Display device comprising first and second transistors electrically connected to first and second pixel electrodes

ABSTRACT

A display device with less light leakage and excellent contrast is provided. A display device having a high aperture ratio and including a large-capacitance capacitor is provided. A display device in which wiring delay due to parasitic capacitance is reduced is provided. A display device includes a transistor over a substrate, a pixel electrode connected to the transistor, a signal line electrically connected to the transistor, a scan line electrically connected to the transistor and intersecting with the signal line, and a common electrode overlapping with the pixel electrode and the signal line with an insulating film provided therebetween. The common electrode includes stripe regions extending in a direction intersecting with the signal line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a driving method thereof, or a manufacturing method thereof. Specifically, one embodiment of the present invention relates to a display device and a manufacturing method thereof.

2. Description of the Related Art

In recent years, liquid crystal has been used for a variety of devices; in particular, a liquid crystal display device (liquid crystal display) having features of thinness and lightness has been used for displays in a wide range of fields.

As a method for applying an electric field to a liquid crystal molecule included in a liquid crystal display device, a vertical electric field mode and a horizontal electric field mode can be given. As a horizontal electric field mode of a liquid crystal display panel, there are an in-plane switching (IPS) mode in which a pixel electrode and a common electrode are formed on the same insulating film and a fringe field switching (FFS) mode in which a pixel electrode and a common electrode overlap with each other with an insulating film provided therebetween.

A liquid crystal display device of an FFS mode has a slit-shaped opening portion in a pixel electrode, and alignment of liquid crystal molecules is controlled by applying an electric field generated between the pixel electrode and a common electrode in the opening portion to the liquid crystal molecules.

The liquid crystal display device of an FFS mode has a high aperture ratio, a wide viewing angle, and an effect of improving an image contrast, and has been widely used recently (see Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2000-89255

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide a display device in which wiring delay due to parasitic capacitance is reduced. Another object of one embodiment of the present invention is to provide a display device with little light leakage and excellent contrast. Another object of one embodiment of the present invention is to provide a display device having a high aperture ratio and including a large-capacitance capacitor. Another object of one embodiment of the present invention is to provide a display device with reduced power consumption. Another object of one embodiment of the present invention is to provide a display device including a transistor having excellent electrical characteristics. Another object of one embodiment of the present invention is to provide a novel display device. Another object of one embodiment of the present invention is to provide a method for manufacturing a display device having a high aperture ratio and a wide viewing angle in fewer steps. Another object of one embodiment of the present invention is to provide a novel method for manufacturing a display device.

Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is a display device including a transistor over an insulating surface, a pixel electrode connected to the transistor, a signal line connected to the transistor, a scan line connected to the transistor and intersecting with the signal line, and a common electrode provided over the pixel electrode and the signal line with an insulating film provided therebetween. The common electrode includes stripe regions extending in a direction intersecting with the signal line.

The transistor includes a gate electrode electrically connected to the scan line, a semiconductor film overlapping with the gate electrode, a gate insulating film between the gate electrode and the semiconductor film, a first conductive film electrically connected to the signal line and the semiconductor film, and a second conductive film electrically connected to the pixel electrode and the semiconductor film. The second conductive film includes a region parallel to the scan line and the stripe regions of the common electrode.

One embodiment of the present invention is a display device including, over an insulating surface, a signal line, a scan line, a transistor, a pixel electrode, a common electrode, and a capacitor. The transistor includes a gate electrode electrically connected to the scan line, a semiconductor film overlapping with the gate electrode, a gate insulating film between the gate electrode and the semiconductor film, a first conductive film electrically connected to the signal line and the semiconductor film, and a second conductive film electrically connected to the pixel electrode and the semiconductor film. The capacitor includes the pixel electrode, the common electrode, and a nitride insulating film provided between the pixel electrode and the common electrode. The common electrode includes stripe regions extending in a direction intersecting with the signal line.

The second conductive film includes a region parallel to the scan line and the stripe regions of the common electrode.

Each of the stripe regions of the common electrode may extend across a plurality of pixel electrodes provided parallel to the scan line.

An angle at which the common electrode and the signal line intersect with each other is preferably larger than or equal to 70° and smaller than or equal to 110°.

The pixel electrodes are provided in a matrix. The common electrode includes a region which intersects with the scan line and is connected to the stripe regions. The semiconductor film and the pixel electrode are in contact with the gate insulating film.

The semiconductor film and the pixel electrode each include an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd).

The semiconductor film and the pixel electrode each have a multilayer structure including a first film and a second film. An atomic ratio of metal elements of the first film is different from that of the second film.

According to one embodiment of the present invention, a display device in which wiring delay due to parasitic capacitance is reduced can be provided. A display device with little light leakage and excellent contrast can be provided. A display device having a high aperture ratio and including a large-capacitance capacitor can be provided. A display device with reduced power consumption can be provided. A display device including a transistor having excellent electrical characteristics can be provided. A display device having a high aperture ratio and a wide viewing angle can be manufactured in fewer steps.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a cross-sectional view and a top view illustrating one embodiment of a display device;

FIGS. 2A to 2D are top views illustrating embodiments of a display device;

FIGS. 3A and 3B are a block diagram and a circuit diagram illustrating one embodiment of a display device;

FIG. 4 is a top view illustrating one embodiment of a display device;

FIG. 5 is a cross-sectional view illustrating one embodiment of a transistor;

FIGS. 6A to 6D are cross-sectional views illustrating one embodiment of a method for manufacturing a transistor;

FIGS. 7A to 7D are cross-sectional views illustrating one embodiment of a method for manufacturing a transistor;

FIGS. 8A to 8C are cross-sectional views illustrating one embodiment of a method for manufacturing a transistor;

FIGS. 9A and 9B are a top view and a cross-sectional view illustrating one embodiment of a display device;

FIG. 10 is a top view illustrating one embodiment of a display device;

FIG. 11 is a top view illustrating one embodiment of a display device;

FIG. 12 is a cross-sectional view illustrating one embodiment of a transistor;

FIGS. 13A to 13C are cross-sectional views illustrating one embodiment of a method for manufacturing a transistor;

FIGS. 14A and 14B are cross-sectional views each illustrating one embodiment of a transistor;

FIG. 15 illustrates a display module;

FIGS. 16A to 16D are each an external view of an electronic appliance of an embodiment;

FIGS. 17A to 17D are top views of Sample 1 and Sample 2 and diagrams showing transmittance distribution thereof;

FIGS. 18A to 18D are top views of Sample 3 and Sample 4 and diagrams showing transmittance distribution thereof;

FIG. 19 is a top view illustrating one embodiment of a display device;

FIG. 20 is a cross-sectional view illustrating one embodiment of a transistor;

FIG. 21 is a cross-sectional view illustrating one embodiment of a transistor;

FIG. 22 is a cross-sectional view illustrating one embodiment of a transistor;

FIG. 23 is a cross-sectional view illustrating one embodiment of a transistor;

FIG. 24 is a top view illustrating one embodiment of a display device;

FIG. 25 is a top view illustrating one embodiment of a display device; and

FIG. 26 is a graph showing temperature dependence of conductivity.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the following description, and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments and examples. In addition, in the following embodiments and examples, the same portions or portions having similar functions are denoted by the same reference numerals or the same hatching patterns in different drawings, and description thereof will not be repeated.

Note that in each drawing described in this specification, the size, the film thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale.

In addition, terms such as “first”, “second”, and “third” in this specification are used in order to avoid confusion among components, and the terms do not limit the components numerically. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate.

Functions of a “source” and a “drain” are sometimes replaced with each other when the direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification.

Note that a voltage refers to a difference between potentials of two points, and a potential refers to electrostatic energy (electric potential energy) of a unit charge at a given point in an electrostatic field. Note that in general, a difference between a potential of one point and a reference potential (e.g., a ground potential) is merely called a potential or a voltage, and a potential and a voltage are used as synonymous words in many cases. Thus, in this specification, a potential may be rephrased as a voltage and a voltage may be rephrased as a potential unless otherwise specified.

Note that in this specification and the like, the term “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

Embodiment 1

In this embodiment, a display device which is one embodiment of the present invention is described with reference to drawings.

FIG. 1A is a cross-sectional view of an FFS mode liquid crystal display device and FIG. 1B is a top view of a pixel 10 in a display portion included in the liquid crystal display device. FIG. 1A corresponds to a cross-sectional view taken along dashed-dotted line A-B in FIG. 1B. In FIG. 1B, a substrate 1, an insulating film 3, an insulating film 8, an insulating film 60, a substrate 61, a light-blocking film 62, a coloring film 63, an insulating film 64, an insulating film 65, and a liquid crystal layer 66 are omitted.

As illustrated in FIGS. 1A and 1B, the FFS mode liquid crystal display device is an active matrix liquid crystal display device and includes a transistor 102 and a pixel electrode 7 in each pixel 10 provided in the display portion.

As illustrated in FIG. 1A, the liquid crystal display device includes the transistor 102 over the substrate 1, the pixel electrode 7 connected to the transistor 102, the insulating film 8 in contact with the transistor 102 and the pixel electrode 7, a common electrode 9 in contact with the insulating film 8, and the insulating film 60 which is in contact with the insulating film 8 and the common electrode 9 and functions as an alignment film.

In addition, the light-blocking film 62 and the coloring film 63 which are in contact with the substrate 61, the insulating film 64 in contact with the substrate 61, the light-blocking film 62, and the coloring film 63, and the insulating film 65 which is in contact with the insulating film 64 and functions as an alignment film are provided. The liquid crystal layer 66 is provided between the insulating film 60 and the insulating film 65. Note that although not illustrated, a polarizing plate is provided outside each of the substrate 1 and the substrate 61.

The transistor 102 can be a staggered transistor, an inverted staggered transistor, a coplanar transistor, or the like as appropriate. In the case of an inverted staggered transistor, a channel-etched structure, a channel protective structure, or the like can be used as appropriate.

The transistor 102 in this embodiment is an inverted staggered transistor having a channel-etched structure. The transistor 102 includes a conductive film 2 functioning as a gate electrode over the substrate 1, the insulating film 3 functioning as a gate insulating film over the substrate 1 and the conductive film 2, a semiconductor film 4 overlapping with the conductive film 2 with the insulating film 3 provided therebetween, and a conductive film 5 and a conductive film 6 both in contact with the semiconductor film 4. The conductive film 2 functions as a scan line as well as functions as a gate electrode. In other words, the gate electrode is part of the scan line. The conductive film 5 functions as a signal line. The conductive films 5 and 6 function as a source electrode and a drain electrode. In other words, one of the source electrode and the drain electrode is part of the signal line. Accordingly, the transistor 102 is electrically connected to the scan line and the signal line. Although the conductive film 2 functions as the gate electrode and the scan line here, the gate electrode and the scan line may be separately formed. The conductive film 5 functions as both the signal line and the one of the source electrode and the drain electrode, but the signal line and the one of the source electrode and the drain electrode may be separately formed.

In the transistor 102, a semiconductor material such as silicon, silicon germanium, or an oxide semiconductor can be used for the semiconductor film 4 as appropriate. The semiconductor film 4 can have an amorphous structure, a microcrystalline structure, a polycrystalline structure, a single crystalline structure, or the like as appropriate.

As illustrated in FIG. 1B, the pixel electrode 7 is rectangular in the pixel 10. Since the display device of this embodiment is an active matrix liquid crystal display device, the pixel electrodes 7 are placed in a matrix. The pixel electrode 7 and the common electrode 9 are each formed using a film having a light-transmitting property.

The shape of the pixel electrode 7 is not limited to a rectangular shape, and can be various shapes in accordance with the shape of the pixel 10. It is preferable that the pixel electrode 7 be widely formed in a region surrounded by the conductive film 2 functioning as a scan line and the conductive film 5 functioning as a signal line in the pixel 10. Thus, the aperture ratio of the pixel 10 can be increased.

The common electrode 9 includes a plurality of regions (first regions) extending in a direction intersecting with the conductive film 5 functioning as a signal line. That is, the common electrode 9 includes stripe regions (a plurality of first regions) extending in a direction intersecting with the conductive film 5 functioning as a signal line. The stripe regions are connected to a region (second region) extending in a direction parallel or substantially parallel to the conductive film 5 functioning as a signal line. That is, the common electrode 9 includes the stripe regions (the plurality of first regions) and the connection region (second region) connected to the stripe regions.

In other words, the common electrode 9 includes, over the pixel electrode 7, the plurality of regions (first regions) extending in a direction parallel or substantially parallel to the conductive film 2 functioning as a scan line. That is, the common electrode 9 includes the stripe regions (the plurality of first regions) extending in a direction parallel or substantially parallel to the conductive film 2 functioning as a scan line. The stripe regions are connected to the region (second region) extending in a direction intersecting with the conductive film 2 functioning as a scan line.

An angle at which a direction in which the stripe regions (the plurality of first regions) of the common electrode 9 extend and a direction in which the conductive film 5 functioning as a signal line extends intersect with each other is preferably larger than or equal to 70° and smaller than or equal to 110°. When the two directions intersect with each other at the angle in the above range, light leakage can be reduced. Further, since the common electrode 9 is not formed over the entire surface of the substrate 1 but includes the stripe regions (the plurality of first regions), parasitic capacitance generated between the common electrode 9 and the conductive film 2 functioning as a scan line and between the common electrode 9 and the conductive film 5 functioning as a signal line can be reduced.

The stripe regions (the plurality of first regions) of the common electrode 9 can each have a linear shape. Alternatively, the stripe regions (the plurality of first regions) of the common electrode 9 may each have a zigzag shape or a wavy shape. In the case where the stripe regions (the plurality of first regions) of the common electrode 9 each have a zigzag shape or a wavy shape, multi-domain alignment of liquid crystal molecules is obtained, and thus the viewing angle can be improved.

Because of the stripe shape of the common electrode 9, a parabolic electric field is generated between the pixel electrode 7 and the common electrode 9 as indicated by dashed arrows in FIG. 1A when voltage is applied to the pixel electrode 7. Accordingly, liquid crystal molecules included in the liquid crystal layer 66 can be aligned.

In a region where the pixel electrode 7 and the common electrode 9 overlap with each other, the pixel electrode 7, the insulating film 8, and the common electrode 9 form a capacitor. Since the pixel electrode 7 and the common electrode 9 are each formed using a film having a light-transmitting property, the aperture ratio and the capacitance of the capacitor can be increased. Furthermore, when the insulating film 8 provided between the pixel electrode 7 and the common electrode 9 is formed using a material having a high dielectric constant, a large amount of charges can be accumulated in the capacitor. As the material having a high dielectric constant, silicon nitride, aluminum oxide, gallium oxide, yttrium oxide, hafnium oxide, hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z)), or the like can be given.

The light-blocking film 62 functions as a black matrix. The coloring film 63 functions as a color filter. The coloring film 63 is not necessarily provided in the case where the liquid crystal display device is a monochrome display device, for example.

The coloring film 63 is a coloring film that transmits light in a specific wavelength range. For example, a red (R) film for transmitting light in a red wavelength range, a green (G) film for transmitting light in a green wavelength range, a blue (B) film for transmitting light in a blue wavelength range, or the like can be used.

The light-blocking film 62 preferably has a function of blocking light in a specific wavelength range, and can be a metal film, an organic insulating film including a black pigment, or the like.

The insulating film 65 functions as a planarization layer or suppresses diffusion of impurities in the coloring film 63 to the liquid crystal element side.

Although not illustrated, a sealant is provided between the substrate 1 and the substrate 61. The liquid crystal layer 66 is enclosed by the substrate 1, the substrate 61, and the sealant. A spacer for keeping the thickness of the liquid crystal layer 66 (also referred to as a cell gap) may be provided between the insulating film 60 and the insulating film 64.

Next, a method for driving the liquid crystal display device of this embodiment is described with reference to FIGS. 2A to 2D.

FIGS. 2A to 2D are each a top view of pixels included in the pixel portion of the FFS mode liquid crystal display device. In each of FIGS. 2A to 2D, two adjacent pixels 10 a and 10 b are shown. In each of FIGS. 2A and 2B, the common electrode 9 extends in a direction parallel or substantially parallel to the conductive film 2 functioning as a scan line. In other words, the common electrode 9 is laid across the pixels 10 a and 10 b.

In each of FIGS. 2A and 2B, the pixels 10 a and 10 b are provided with the common electrode 9 including stripe regions extending in a direction intersecting with conductive films 5 a and 5 b functioning as a signal line. In each of FIGS. 2C and 2D, the pixels 10 a and 10 b are provided with the common electrode 9 including stripe regions extending in a direction intersecting with the conductive film 2 functioning as a scan line. A method for driving a display element in a pixel, in which black display in an initial state is turned into white display by application of voltage to a pixel electrode, i.e., a method for driving a display element of a normally black mode, is described. Note that a display element here is the pixel electrode 7, the common electrode 9, and a liquid crystal molecule included in the liquid crystal layer. Although a method for driving a display element of a normally black mode is described in this embodiment, a method for driving a display element of a normally white mode can be used as appropriate.

In the case of black display, voltage at which a transistor is turned on is applied to a scan line, and 0 V is applied to a signal line and a common electrode. As a result, 0 V is applied to the pixel electrode. In other words, an electric field is not generated between the pixel electrode and the common electrode, and thus liquid crystal molecules do not operate.

In the case of white display, voltage at which a transistor is turned on is applied to a scan line, voltage at which liquid crystal molecules operate, e.g., 6 V, is applied to a signal line, and 0 V is applied to a common electrode. As a result, 6 V is applied to the pixel electrode. In other words, an electric field is generated between the pixel electrode and the common electrode, and thus the liquid crystal molecules operate.

Here, a negative liquid crystal material is used in this description. The liquid crystal molecules are aligned in a direction perpendicular to the common electrode in an initial state. The alignment of the liquid crystal molecules in an initial state is referred to as initial alignment. The liquid crystal molecules rotate in a plane parallel to a substrate by application of voltage between the pixel electrode and the common electrode. Although the negative liquid crystal material is used in this embodiment, a positive liquid crystal material can be used as appropriate.

The polarizing plate is provided outside each of the substrate 1 and the substrate 61 in FIG. 1A. A polarizer of the polarizing plate provided outside the substrate 1 and a polarizer of the polarizing plate provided outside the substrate 61 are placed to intersect with each other at right angles, that is, placed in a crossed Nicols state. Therefore, when the liquid crystal molecules are aligned in a direction parallel to the conductive film 2 functioning as a scan line or the conductive films 5 a and 5 b functioning as a signal line, light is absorbed by the polarizing plates and black is displayed. Although the polarizers are placed in a crossed Nicols state in this embodiment, the polarizers can be placed in a parallel Nicols state as appropriate.

In each of FIGS. 2A to 2D, the pixel 10 a includes the conductive film 2 functioning as a scan line, a semiconductor film 4 a, a conductive film 5 a functioning as a signal line, a conductive film 6 a, a pixel electrode 7 a, and the common electrode 9, and the pixel 10 b includes the conductive film 2 functioning as a scan line, a semiconductor film 4 b, a conductive film 5 b functioning as a signal line, a conductive film 6 b, a pixel electrode 7 b, and the common electrode 9. FIGS. 2A and 2C each illustrate an initial state and FIGS. 2B and 2D each illustrate a state where the pixel 10 b performs white display.

Since the common electrode 9 included in the pixels 10 a and 10 b in each of FIGS. 2C and 2D extends in a direction parallel or substantially parallel to the conductive films 5 a and 5 b functioning as signal lines, liquid crystal molecules L are aligned in a direction perpendicular to the conductive films 5 a and 5 b functioning as signal lines in an initial state (black display) illustrated in FIG. 2C.

The case where the pixel 10 a performs black display and the pixel 10 b performs white display, as in FIG. 2D, is described. To the common electrode 9 and the conductive film 5 a functioning as a signal line is applied 0 V. To the conductive film 5 b functioning as a signal line is applied 6 V. As a result, 6 V is applied to the pixel electrode 7 b in the pixel 10 b, an electric field as indicated by arrows in FIG. 2D is generated between the pixel electrode 7 b and the common electrode 9, and the liquid crystal molecules L are aligned accordingly. Here, the liquid crystal molecules L rotate by 45°.

A potential of the pixel electrode 7 a is 0 V in the pixel 10 a and a potential of the conductive film 5 b functioning as a signal line, which is provided in the vicinity of the pixel electrode 7 a, is 6 V. Therefore, also in the pixel 10 a, an electric field as indicated by an arrow in FIG. 2D is generated between the pixel electrode 7 a and the conductive film 5 b functioning as a signal line, and the liquid crystal molecules L are aligned accordingly. As a result, in the pixel 10 a where black display should be performed, alignment of some of the liquid crystal molecules L is changed, causing light leakage.

In contrast, in the pixels 10 a and 10 b in each of FIGS. 2A and 2B, the common electrode 9 extends in a direction perpendicular to the conductive films 5 a and 5 b functioning as signal lines; therefore, the liquid crystal molecules L are aligned in a direction parallel or substantially parallel to the conductive films 5 a and 5 b functioning as signal lines in an initial state (black display).

The case where the pixel 10 a performs black display and the pixel 10 b performs white display, as in FIG. 2B, is described. To the common electrode 9 and the conductive film 5 a functioning as a signal line is applied 0 V. To the conductive film 5 b functioning as a signal line is applied 6 V. As a result, 6 V is applied to the pixel electrode 7 b in the pixel 10 b, an electric field as indicated by arrows in FIG. 2B is generated between the pixel electrode 7 b and the common electrode 9, and the liquid crystal molecules L are aligned accordingly. Here, the liquid crystal molecules L rotate by −45°.

The potential of the pixel electrode 7 a is 0 V in the pixel 10 a and the potential of the conductive film 5 b functioning as a signal line, which is provided in the vicinity of the pixel electrode 7 a, is 6 V. However, since the common electrode 9 and the conductive film 5 b functioning as a signal line intersect with each other, a first electric field F1 generated between the pixel electrode 7 a and the conductive film 5 b functioning as a signal line and a major axis of the liquid crystal molecule L intersect with each other at right angles. As a result, the liquid crystal molecule L, which is included in a negative liquid crystal material, does not operate and thus light leakage can be suppressed.

For the above-described reason, when a common electrode extending in a direction intersecting with a signal line is provided in an FFS mode liquid crystal display device, the display device can have excellent contrast.

The common electrode 9 of this embodiment is not formed over the entire surface of the substrate. Therefore, a region where the common electrode 9 overlaps with the conductive films 5 a and 5 b functioning as signal lines can be reduced and thus parasitic capacitance generated between the signal line and the common electrode 9 can be reduced. As a result, wiring delay can be reduced in a display device formed using a large substrate.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a display device which is one embodiment of the present invention is described with reference to drawings. In addition, in this embodiment, an oxide semiconductor film is used as a semiconductor film included in a transistor.

A display device illustrated in FIG. 3A includes a pixel portion 101; a scan line driver circuit 104; a signal line driver circuit 106; m scan lines 107 which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the scan line driver circuit 104; and n signal lines 109 which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the signal line driver circuit 106. Furthermore, the pixel portion 101 includes a plurality of pixels 103 arranged in a matrix. Furthermore, common lines 115 arranged parallel or substantially parallel to each other are provided along the signal lines 109. The scan line driver circuit 104 and the signal line driver circuit 106 are collectively referred to as a driver circuit portion in some cases.

Each scan line 107 is electrically connected to the n pixels 103 in the corresponding row among the pixels 103 arranged in m rows and n columns in the pixel portion 101. Each signal line 109 is electrically connected to the m pixels 103 in the corresponding column among the pixels 103 arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each common line 115 is electrically connected to the m pixels 103 in the corresponding column among the pixels 103 arranged in m rows and n columns.

FIG. 3B illustrates an example of a circuit configuration that can be used for the pixels 103 in the display device illustrated in FIG. 3A.

The pixel 103 in FIG. 3B includes a liquid crystal element 121, a transistor 102, and a capacitor 105.

One of a pair of electrodes of the liquid crystal element 121 is connected to the transistor 102 and the potential thereof is set according to the specifications of the pixel 103 as appropriate. The other of the pair of electrodes of the liquid crystal element 121 is connected to the common line 115 and a common potential is applied thereto. The alignment of liquid crystal molecules of the liquid crystal element 121 is controlled in accordance with data written to the transistor 102.

The liquid crystal element 121 is an element that controls transmission or non-transmission of light utilizing an optical modulation action of a liquid crystal molecule. Note that the optical modulation action of the liquid crystal molecule is controlled by an electric field applied to the liquid crystal molecule (including a horizontal electric field, a vertical electric field, and a diagonal electric field). Examples of a liquid crystal material used for the liquid crystal element 121 are a nematic liquid crystal, a cholesteric liquid crystal, a smectic liquid crystal, a thermotropic liquid crystal, a lyotropic liquid crystal, a ferroelectric liquid crystal, and an anti-ferroelectric liquid crystal.

An FFS mode is used as a method for driving the display device including the liquid crystal element 121.

The liquid crystal element may be formed using a liquid crystal composition including a liquid crystal material exhibiting a blue phase and a chiral material. The liquid crystal exhibiting a blue phase has a short response time of 1 msec or less and is optically isotropic; therefore, alignment treatment is not necessary and viewing angle dependence is small.

In the structure of the pixel 103 illustrated in FIG. 3B, one of a source electrode and a drain electrode of the transistor 102 is electrically connected to the signal line 109, and the other is electrically connected to the one of the pair of electrodes of the liquid crystal element 121. A gate electrode of the transistor 102 is electrically connected to the scan line 107. The transistor 102 has a function of controlling whether to write a data signal by being turned on or off.

In the pixel 103 in FIG. 3B, one of a pair of electrodes of the capacitor 105 is connected to the transistor 102. The other of the pair of electrodes of the capacitor 105 is electrically connected to the common line 115. The potential of the common line 115 is set in accordance with the specifications of the pixel 103 as appropriate. The capacitor 105 functions as a storage capacitor for storing written data. In this embodiment, the one of the pair of electrodes of the capacitor 105 is the one of the pair of electrodes of the liquid crystal element 121. The other of the pair of electrodes of the capacitor 105 is the other of the pair of electrodes of the liquid crystal element 121.

A specific structure of an element substrate included in the display device is described. FIG. 4 is a top view of a plurality of pixels 103 a, 103 b, and 103 c.

In FIG. 4, a conductive film 13 functioning as a scan line extends in a direction substantially perpendicularly to the signal line (in the horizontal direction in the drawing). A conductive film 21 a functioning as a signal line extends in a direction substantially perpendicularly to the scan line (in the vertical direction in the drawing). Note that the conductive film 13 functioning as a scan line is electrically connected to the scan line driver circuit 104 (see FIG. 3A), and the conductive film 21 a functioning as a signal line is electrically connected to the signal line driver circuit 106 (see FIG. 3A).

The transistor 102 is provided at a region where the scan line and the signal line intersect with each other. The transistor 102 includes the conductive film 13 functioning as a gate electrode; a gate insulating film (not illustrated in FIG. 4); an oxide semiconductor film 19 a where a channel region is formed, over the gate insulating film; and the conductive film 21 a and a conductive film 21 b functioning as a source electrode and a drain electrode. The conductive film 13 also functions as a scan line, and a region of the conductive film 13 that overlaps with the oxide semiconductor film 19 a serves as the gate electrode of the transistor 102. In addition, the conductive film 21 a also functions as a signal line, and a region of the conductive film 21 a that overlaps with the oxide semiconductor film 19 a functions as the source electrode or the drain electrode of the transistor 102. Furthermore, in the top view of FIG. 4, an end portion of the scan line is located on the outer side of an end portion of the oxide semiconductor film 19 a. Thus, the scan line functions as a light-blocking film for blocking light from a light source such as a backlight. For this reason, the oxide semiconductor film 19 a included in the transistor is not irradiated with light, so that a variation in the electrical characteristics of the transistor can be suppressed.

The conductive film 21 b is electrically connected to the pixel electrode 19 b. A common electrode 29 is provided over the pixel electrode 19 b with an insulating film provided therebetween. An opening portion 40 indicated by a dashed-dotted line is provided in the insulating film provided over the pixel electrode 19 b. The pixel electrode 19 b is in contact with a nitride insulating film (not illustrated in FIG. 4) in the opening portion 40.

The common electrode 29 includes stripe regions (a plurality of first regions) extending in a direction intersecting with a signal line. The plurality of first regions is connected to a second region extending in a direction parallel or substantially parallel to a signal line. Accordingly, the stripe regions (the plurality of first regions) of the common electrode 29 are at the same potential.

The capacitor 105 is formed in a region where the pixel electrode 19 b and the common electrode 29 overlap with each other. The pixel electrode 19 b and the common electrode 29 each have a light-transmitting property. That is, the capacitor 105 has a light-transmitting property.

As illustrated in FIG. 4, the liquid crystal display device described in this embodiment is an FFS mode liquid crystal display device and is provided with the common electrode 29 including the stripe regions extending in a direction intersecting with a signal line. Thus, the display device can have excellent contrast.

Owing to the light-transmitting property of the capacitor 105, the capacitor 105 can be formed large (in a large area) in the pixel 103. Thus, a display device with a large-capacitance capacitor as well as an aperture ratio increased to typically 50% or more, preferably 60% or more can be provided. For example, in a high-resolution display device such as a liquid crystal display device, the area of a pixel is small and accordingly the area of a capacitor is also small. For this reason, the amount of charges accumulated in the capacitor is small in the high-resolution display device. However, since the capacitor 105 of this embodiment has a light-transmitting property, when the capacitor 105 is provided in a pixel, enough capacitance can be obtained in the pixel and the aperture ratio can be improved. Typically, the capacitor 105 can be favorably used for a high-resolution display device with a pixel density of 200 pixels per inch (ppi) or more, 300 ppi or more, or furthermore, 500 ppi or more.

In a liquid crystal display device, as the capacitance value of a capacitor is increased, a period during which the alignment of liquid crystal molecules of a liquid crystal element can be kept constant in the state where an electric field is applied can be made longer. When the period can be made longer in a display device which displays a still image, the number of times of rewriting image data can be reduced, leading to a reduction in power consumption. Further, according to the structure of this embodiment, the aperture ratio can be improved even in a high-resolution display device, which makes it possible to use light from a light source such as a backlight efficiently, so that power consumption of the display device can be reduced.

Note that a top view of one embodiment of the present invention is not limited to FIG. 4. The display device can have a variety of different structures. For example, connection regions of the common electrode 29 may be formed over conductive films functioning as signal lines as illustrated in FIG. 19.

Next, FIG. 5 is a cross-sectional view taken along dashed-dotted lines A-B and C-D in FIG. 4. The transistor 102 shown in FIG. 5 is a channel-etched transistor. Note that the transistor 102 in the channel length direction and the capacitor 105 are illustrated in the cross-sectional view taken along dashed-dotted line A-B, and the transistor 102 in the channel width direction is illustrated in the cross-sectional view taken along dashed-dotted line C-D.

The transistor 102 in FIG. 5 has a single-gate structure and includes the conductive film 13 functioning as a gate electrode over the substrate 11. In addition, the transistor 102 includes a nitride insulating film 15 formed over the substrate 11 and the conductive film 13 functioning as a gate electrode, an oxide insulating film 17 formed over the nitride insulating film 15, the oxide semiconductor film 19 a overlapping with the conductive film 13 functioning as a gate electrode with the nitride insulating film 15 and the oxide insulating film 17 provided therebetween, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode which are in contact with the oxide semiconductor film 19 a. Moreover, an oxide insulating film 23 is formed over the oxide insulating film 17, the oxide semiconductor film 19 a, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, and an oxide insulating film 25 is formed over the oxide insulating film 23. A nitride insulating film 27 is formed over the oxide insulating film 23, the oxide insulating film 25, and the conductive film 21 b. The pixel electrode 19 b is formed over the oxide insulating film 17. The pixel electrode 19 b is connected to one of the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, here, connected to the conductive film 21 b. The common electrode 29 is formed over the nitride insulating film 27.

A region where the pixel electrode 19 b, the nitride insulating film 27, and the common electrode 29 overlap with one another functions as the capacitor 105.

Note that a cross-sectional view of one embodiment of the present invention is not limited to FIG. 5. The display device can have a variety of different structures. For example, the pixel electrode 19 b may have a slit. The pixel electrode 19 b may have a comb-like shape. An example of a cross-sectional view in this case is shown in FIG. 20. Alternatively, an insulating film 26 b may be provided over the nitride insulating film 27 as illustrated in FIG. 21. For example, an organic resin film may be provided as the insulating film 26 b. Thus, the insulating film 26 b can have a flat surface. In other words, as an example, the insulating film 26 b can function as a planarization film. Alternatively, a capacitor 105 b may be formed so that the common electrode 29 and the conductive film 21 b overlap with each other. Examples of a cross-sectional view in this case are shown in FIG. 22 and FIG. 23.

A structure of the display device is described below in detail.

There is no particular limitation on the property of a material and the like of the substrate 11 as long as the material has heat resistance enough to withstand at least later heat treatment. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 11. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI (silicon on insulator) substrate, or the like may be used as the substrate 11. Furthermore, any of these substrates further provided with a semiconductor element may be used as the substrate 11. In the case where a glass substrate is used as the substrate 11, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 11, and the transistor 102 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 11 and the transistor 102. The separation layer can be used when part or the whole of a display device formed over the separation layer is separated from the substrate 11 and transferred onto another substrate. In such a case, the transistor 102 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

The conductive film 13 functioning as a gate electrode can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing any of these metal elements in combination; or the like. Further, one or more metal elements selected from manganese and zirconium may be used. The conductive film 13 functioning as a gate electrode may have a single-layer structure or a stacked structure of two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a two-layer structure in which a copper film is stacked over a titanium film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, an alloy film or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used.

The conductive film 13 functioning as a gate electrode can also be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked structure formed using the above light-transmitting conductive material and the above metal element.

The nitride insulating film 15 can be a nitride insulating film that is hardly permeated by oxygen. Furthermore, a nitride insulating film that is hardly permeated by oxygen, hydrogen, and water can be used. As the nitride insulating film that is hardly permeated by oxygen and the nitride insulating film that is hardly permeated by oxygen, hydrogen, and water, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like is given. Instead of the nitride insulating film that is hardly permeated by oxygen and the nitride insulating film that is hardly permeated by oxygen, hydrogen, and water, an oxide insulating film such as an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, or a hafnium oxynitride film can be used.

The thickness of the nitride insulating film 15 is preferably greater than or equal to 5 nm and less than or equal to 100 nm, more preferably greater than or equal to 20 nm and less than or equal to 80 nm.

The oxide insulating film 17 may be formed to have a single-layer structure or a stacked structure using, for example, one or more of a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, a hafnium oxide film, a gallium oxide film, a Ga—Zn-based metal oxide film, and a silicon nitride film.

The oxide insulating film 17 may also be formed using a material having a high relative dielectric constant such as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced.

The thickness of the oxide insulating film 17 is preferably greater than or equal to 5 nm and less than or equal to 400 nm, more preferably greater than or equal to 10 nm and less than or equal to 300 nm, further preferably greater than or equal to 50 nm and less than or equal to 250 nm.

The oxide semiconductor film 19 a is typically formed using In—Ga oxide, In—Zn oxide, or In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd).

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxide film, the proportions of In and M when summation of In and M is assumed to be 100 atomic % are preferably as follows: the atomic percentage of In is greater than 25 atomic % and the atomic percentage of M is less than 75 atomic %, or more preferably, the atomic percentage of In is greater than 34 atomic % and the atomic percentage of M is less than 66 atomic %.

The energy gap of the oxide semiconductor film 19 a is 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more. The off-state current of the transistor 102 can be reduced by using an oxide semiconductor having such a wide energy gap.

The thickness of the oxide semiconductor film 19 a is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, more preferably greater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd), it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide film satisfy In≥M and Zn≥M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Note that the proportion of each metal element in the atomic ratio of the oxide semiconductor film 19 a to be formed varies within a range of ±40% of that in the above atomic ratio of the sputtering target as an error.

An oxide semiconductor film with low carrier density is used as the oxide semiconductor film 19 a. For example, an oxide semiconductor film whose carrier density is 1×10¹⁷/cm³ or lower, preferably 1×10¹⁵/cm³ or lower, more preferably 1×10¹³/cm³ or lower, much more preferably 1×10¹¹/cm³ or lower is used as the oxide semiconductor film 19 a.

Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Further, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor film 19 a be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 19 a, an oxide semiconductor film in which the impurity concentration is low and density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which impurity concentration is low and density of defect states is low (the amount of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus has a low carrier density in some cases. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases. Further, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10⁶ μm and a channel length (L) of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. Thus, the transistor in which a channel region is formed in the oxide semiconductor film has a small variation in electrical characteristics and high reliability in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given.

Hydrogen contained in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and in addition, an oxygen vacancy is formed in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Further, in some cases, bonding of part of hydrogen to oxygen bonded to a metal element causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor which contains hydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much as possible as well as the oxygen vacancies in the oxide semiconductor film 19 a. Specifically, in the oxide semiconductor film 19 a, the concentration of hydrogen which is measured by secondary ion mass spectrometry (SIMS) is set to lower than or equal to 5×10¹⁹ atoms/cm³, preferably lower than or equal to 1×10¹⁹ atoms/cm³, more preferably lower than or equal to 5×10¹⁸ atoms/cm³, still more preferably lower than or equal to 1×10¹⁸ atoms/cm³, yet more preferably lower than or equal to 5×10¹⁷ atoms/cm³, further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon which is one of elements belonging to Group 14 is contained in the oxide semiconductor film 19 a, oxygen vacancies are increased in the oxide semiconductor film 19 a, and the oxide semiconductor film 19 a becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) of the oxide semiconductor film 19 a is set to lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

The concentration of alkali metal or alkaline earth metal in the oxide semiconductor film 19 a, which is measured by SIMS, is set to lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film 19 a.

Further, when containing nitrogen, the oxide semiconductor film 19 a easily has n-type conductivity by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor which contains nitrogen is likely to be normally on. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 19 a may have a non-single-crystal structure, for example. The non-single-crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure which is described later, or an amorphous structure, for example. Among the non-single-crystal structures, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states.

The oxide semiconductor film 19 a may have an amorphous structure, for example. An oxide semiconductor film having an amorphous structure has disordered atomic arrangement and no crystalline component, for example.

Note that the oxide semiconductor film 19 a may be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. The mixed film has a single-layer structure including, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Further, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases.

The pixel electrode 19 b is formed by processing an oxide semiconductor film formed at the same time as the oxide semiconductor film 19 a. Thus, the pixel electrode 19 b contains a metal element similar to that in the oxide semiconductor film 19 a. Further, the pixel electrode 19 b has a crystal structure similar to or different from that of the oxide semiconductor film 19 a. By adding impurities or oxygen vacancies to the oxide semiconductor film formed at the same time as the oxide semiconductor film 19 a, the oxide semiconductor film has conductivity and thus functions as the pixel electrode 19 b. An example of the impurities contained in the oxide semiconductor film is hydrogen. Instead of hydrogen, as the impurity, boron, phosphorus, tin, antimony, a rare gas element, an alkali metal, an alkaline earth metal, or the like may be included. Alternatively, the pixel electrode 19 b is formed at the same time as the oxide semiconductor film 19 a, and has increased conductivity by containing oxygen vacancies generated by plasma damage or the like. Alternatively, the pixel electrode 19 b is formed at the same time as the oxide semiconductor film 19 a, and has increased conductivity by containing impurities and oxygen vacancies generated by plasma damage or the like.

The oxide semiconductor film 19 a and the pixel electrode 19 b are both formed over the oxide insulating film 17, but differ in impurity concentration. Specifically, the pixel electrode 19 b has a higher impurity concentration than the oxide semiconductor film 19 a. For example, the concentration of hydrogen contained in the oxide semiconductor film 19 a is lower than or equal to 5×10¹⁹ atoms/cm³, preferably lower than or equal to 1×10¹⁹ atoms/cm³, more preferably lower than or equal to 5×10¹⁸ atoms/cm³, still more preferably lower than or equal to 1×10¹⁸ atoms/cm³, yet more preferably lower than or equal to 5×10¹⁷ atoms/cm³, further preferably lower than or equal to 1×10¹⁶ atoms/cm³. The concentration of hydrogen contained in the pixel electrode 19 b is higher than or equal to 8×10¹⁹ atoms/cm³, preferably higher than or equal to 1×10²⁰ atoms/cm³, more preferably higher than or equal to 5×10²⁰ atoms/cm³. The concentration of hydrogen contained in the pixel electrode 19 b is greater than or equal to 2 times, preferably greater than or equal to 10 times that in the oxide semiconductor film 19 a.

When the oxide semiconductor film formed at the same time as the oxide semiconductor film 19 a is exposed to plasma, the oxide semiconductor film is damaged, and oxygen vacancies can be generated. For example, when a film is formed over the oxide semiconductor film by a plasma CVD method or a sputtering method, the oxide semiconductor film is exposed to plasma and oxygen vacancies are generated. Alternatively, when the oxide semiconductor film is exposed to plasma in etching treatment for formation of the oxide insulating film 23 and the oxide insulating film 25, oxygen vacancies are generated. Alternatively, when the oxide semiconductor film is exposed to plasma of a mixed gas of oxygen and hydrogen, hydrogen, a rare gas, ammonia, or the like, oxygen vacancies are generated. As a result, the conductivity of the oxide semiconductor film is increased, so that the oxide semiconductor film functions as the pixel electrode 19 b.

In other words, the pixel electrode 19 b is formed using an oxide semiconductor film having high conductivity. It can also be said that the pixel electrode 19 b is formed using a metal oxide film having high conductivity.

In the case where a silicon nitride film is used as the nitride insulating film 27, the silicon nitride film contains hydrogen. When hydrogen in the nitride insulating film 27 is diffused into the oxide semiconductor film formed at the same time as the oxide semiconductor film 19 a, hydrogen is bonded to oxygen and electrons serving as carriers are generated in the oxide semiconductor film. When the silicon nitride film is formed by a plasma CVD method or a sputtering method, the oxide semiconductor film is exposed to plasma and oxygen vacancies are generated in the oxide semiconductor film. When hydrogen contained in the silicon nitride film enters the oxygen vacancies, electrons serving as carriers are generated. As a result, the conductivity of the oxide semiconductor film is increased, so that the oxide semiconductor film functions as the pixel electrode 19 b.

When hydrogen is added to an oxide semiconductor including oxygen vacancies, hydrogen enters oxygen vacant sites and forms a donor level in the vicinity of the conduction band. As a result, the conductivity of the oxide semiconductor is increased, so that the oxide semiconductor becomes a conductor. An oxide semiconductor having become a conductor can be referred to as an oxide conductor. In other words, the pixel electrode 19 b is formed using an oxide conductor film. Oxide semiconductors generally have a visible light-transmitting property because of their large energy gap. An oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band. Therefore, the influence of absorption due to the donor level is small, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

The pixel electrode 19 b has lower resistivity than the oxide semiconductor film 19 a. The resistivity of the pixel electrode 19 b is preferably greater than or equal to 1×10⁻⁸ times and less than 1×10⁻¹ times the resistivity of the oxide semiconductor film 19 a. The resistivity of the pixel electrode 19 b is typically greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, preferably greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

The conductive films 21 a and 21 b functioning as a source electrode and a drain electrode are each formed to have a single-layer structure or a stacked-layer structure including any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten or an alloy containing any of these metals as its main component. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and the like can be given. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used.

As the oxide insulating film 23 or the oxide insulating film 25, an oxide insulating film which contains more oxygen than that in the stoichiometric composition is preferably used. Here, as the oxide insulating film 23, an oxide insulating film which is permeated by oxygen is formed, and as the oxide insulating film 25, an oxide insulating film which contains more oxygen than that in the stoichiometric composition is formed.

The oxide insulating film 23 is an oxide insulating film which is permeated by oxygen. Thus, oxygen released from the oxide insulating film 25 provided over the oxide insulating film 23 can be moved to the oxide semiconductor film 19 a through the oxide insulating film 23. Moreover, the oxide insulating film 23 also functions as a film that relieves damage to the oxide semiconductor film 19 a at the time of forming the oxide insulating film 25 later.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used as the oxide insulating film 23. Note that in this specification, “silicon oxynitride film” refers to a film that contains oxygen at a higher proportion than nitrogen, and “silicon nitride oxide film” refers to a film that contains nitrogen at a higher proportion than oxygen.

Further, it is preferable that the amount of defects in the oxide insulating film 23 be small and typically, the spin density of a signal that appears at g=2.001 be lower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement. The signal that appears at g=2.001 is due to dangling bonds of silicon. This is because if the density of defects in the oxide insulating film 23 is high, oxygen is bonded to the defects and the amount of oxygen that passes through the oxide insulating film 23 is decreased.

Further, it is preferable that the amount of defects at the interface between the oxide insulating film 23 and the oxide semiconductor film 19 a be small and typically, the spin density of a signal that appears at g=1.93 due to an oxygen vacancy in the oxide semiconductor film 19 a be lower than or equal to 1×10¹⁷ spins/cm³, more preferably lower than or equal to the lower limit of detection by ESR measurement.

Note that in the oxide insulating film 23, all oxygen that enters the oxide insulating film 23 from the outside is transferred to the outside of the oxide insulating film 23 in some cases. Alternatively, some oxygen that enters the oxide insulating film 23 from the outside remains in the oxide insulating film 23 in some cases. Further, movement of oxygen occurs in the oxide insulating film 23 in some cases in such a manner that oxygen enters the oxide insulating film 23 from the outside and oxygen contained in the oxide insulating film 23 is transferred to the outside of the oxide insulating film 23.

The oxide insulating film 25 is formed in contact with the oxide insulating film 23. The oxide insulating film 25 is formed using an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition. The oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used as the oxide insulating film 25.

It is preferable that the amount of defects in the oxide insulating film 25 be small and typically, the spin density of a signal that appears at g=2.001 be lower than 1.5×10¹⁸ spins/cm³, more preferably lower than or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the oxide insulating film 25 is provided more apart from the oxide semiconductor film 19 a than the oxide insulating film 23 is; thus, the oxide insulating film 25 may have higher defect density than the oxide insulating film 23.

Like the nitride insulating film 15, the nitride insulating film 27 can be a nitride insulating film which is hardly permeated by oxygen. Furthermore, a nitride insulating film which is hardly permeated by oxygen, hydrogen, and water can be used.

The nitride insulating film 27 is formed using a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like with a thickness greater than or equal to 50 nm and less than or equal to 300 nm, preferably greater than or equal to 100 nm and less than or equal to 200 nm.

In the case where the oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition is included in the oxide insulating film 23 or the oxide insulating film 25, part of oxygen contained in the oxide insulating film 23 or the oxide insulating film 25 can be transferred to the oxide semiconductor film 19 a, so that the amount of oxygen vacancies contained in the oxide semiconductor film 19 a can be reduced.

The threshold voltage of a transistor using an oxide semiconductor film with oxygen vacancies easily shifts negatively, and such a transistor tends to be normally on. This is because charges are generated owing to oxygen vacancies in the oxide semiconductor film and the resistance is thus reduced. The transistor having normally-on characteristics causes various problems in that malfunction is likely to be caused when in operation and that power consumption is increased when not in operation, for example. Further, there is a problem in that the amount of change in electrical characteristics, typically in threshold voltage, of the transistor is increased by change over time or a stress test.

However, in the transistor 102 in this embodiment, the oxide insulating film 23 or the oxide insulating film 25 provided over the oxide semiconductor film 19 a contains oxygen at a higher proportion than the stoichiometric composition. As a result, oxygen contained in the oxide insulating film 23 or the oxide insulating film 25 is moved to the oxide semiconductor film 19 a efficiently, so that the amount of oxygen vacancies in the oxide semiconductor film 19 a can be reduced. Accordingly, a transistor having normally-off characteristics is obtained. Further, the amount of change in electrical characteristics, typically in threshold voltage, of the transistor over time or due to a stress test can be reduced.

The common electrode 29 is formed using a light-transmitting conductive film. As the light-transmitting conductive film, an indium oxide film containing tungsten oxide, an indium zinc oxide film containing tungsten oxide, an indium oxide film containing titanium oxide, an indium tin oxide film containing titanium oxide, an indium tin oxide (hereinafter, referred to as ITO) film, an indium zinc oxide film, an indium tin oxide film to which silicon oxide is added, and the like are given.

The common electrode 29 includes the stripe regions extending in a direction intersecting with the conductive film 21 a functioning as a signal line. Accordingly, in the vicinity of the pixel electrode 19 b and the conductive film 21 a, unintended alignment of liquid crystal molecules can be prevented and thus light leakage can be suppressed. As a result, a display device with excellent contrast can be manufactured.

On an element substrate of the display device described in this embodiment, the pixel electrode is formed at the same time as the oxide semiconductor film of the transistor. The pixel electrode also functions as one of electrodes of the capacitor. The common electrode also functions as the other of electrodes of the capacitor. Thus, a step of forming another conductive film is not needed to form the capacitor, and the number of steps of manufacturing the display device can be reduced. The capacitor has a light-transmitting property. As a result, the area occupied by the capacitor can be increased and the aperture ratio in a pixel can be increased.

Next, a method for manufacturing the transistor 102 and the capacitor 105 in FIG. 5 is described with reference to FIGS. 6A to 6D, FIGS. 7A to 7D, and FIGS. 8A to 8C.

As illustrated in FIG. 6A, a conductive film 12 to be the conductive film 13 is formed over the substrate 11. The conductive film 12 is formed by a sputtering method, a chemical vapor deposition (CVD) method such as a metal organic chemical vapor deposition (MOCVD) method, a metal chemical vapor deposition method, an atomic layer deposition (ALD) method, or a plasma-enhanced chemical vapor deposition (PECVD) method, an evaporation method, a pulsed laser deposition (PLD) method, or the like. When a metal organic chemical vapor deposition (MOCVD) method, a metal chemical vapor deposition method, or an atomic layer deposition (ALD) method is employed, the conductive film is less damaged by plasma.

Here, a glass substrate is used as the substrate 11. Further, as the conductive film 12, a 100-nm-thick tungsten film is formed by a sputtering method.

Then, a mask is formed over the conductive film 12 by a photolithography process using a first photomask. Next, as illustrated in FIG. 6B, part of the conductive film 12 is etched with the use of the mask to form the conductive film 13 functioning as a gate electrode. After that, the mask is removed.

Note that the conductive film 13 functioning as a gate electrode may be formed by an electrolytic plating method, a printing method, an ink jet method, or the like instead of the above formation method.

Here, the tungsten film is etched by dry etching to form the conductive film 13 functioning as a gate electrode.

Next, as illustrated in FIG. 6C, over the conductive film 13 functioning as a gate electrode, the nitride insulating film 15 and an oxide insulating film 16 to be the oxide insulating film 17 later are formed. Then, over the oxide insulating film 16, an oxide semiconductor film 18 to be the oxide semiconductor film 19 a and the pixel electrode 19 b later is formed.

The nitride insulating film 15 and the oxide insulating film 16 are each formed by a sputtering method, a chemical vapor deposition (CVD) method such as a metal organic chemical vapor deposition (MOCVD) method, a metal chemical vapor deposition method, an atomic layer deposition (ALD) method, or a plasma-enhanced chemical vapor deposition (PECVD) method, an evaporation method, a pulsed laser deposition (PLD) method, a coating method, a printing method, or the like. When a metal organic chemical vapor deposition (MOCVD) method, a metal chemical vapor deposition method, or an atomic layer deposition (ALD) method is employed, the nitride insulating film 15 and the oxide insulating film 16 are less damaged by plasma. When an atomic layer deposition (ALD) method is employed, coverage of the nitride insulating film 15 and the oxide insulating film 16 can be increased.

Here, as the nitride insulating film 15, a 300-nm-thick silicon nitride film is formed by a plasma CVD method in which silane, nitrogen, and ammonia are used as a source gas.

In the case where a silicon oxide film, a silicon oxynitride film, or a silicon nitride oxide film is formed as the oxide insulating film 16, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given as examples.

Moreover, in the case of forming a gallium oxide film as the oxide insulating film 16, an MOCVD method can be employed.

Here, as the oxide insulating film 16, a 50-nm-thick silicon oxynitride film is formed by a plasma CVD method in which silane and dinitrogen monoxide are used as a source gas.

The oxide semiconductor film 18 can be formed by a sputtering method, a chemical vapor deposition (CVD) method such as a metal organic chemical vapor deposition (MOCVD) method, an atomic layer deposition (ALD) method, or a plasma-enhanced chemical vapor deposition (PECVD) method, a pulsed laser deposition method, a laser ablation method, a coating method, or the like. When a metal organic chemical vapor deposition (MOCVD) method or an atomic layer deposition (ALD) method is employed, the oxide semiconductor film 18 is less damaged by plasma and the oxide insulating film 16 is less damaged. When an atomic layer deposition (ALD) method is employed, coverage of the oxide semiconductor film 18 can be increased.

As a power supply device for generating plasma in the case of forming the oxide semiconductor film by a sputtering method, an RF power supply device, an AC power supply device, a DC power supply device, or the like can be used as appropriate.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased.

Further, a target may be selected as appropriate in accordance with the composition of the oxide semiconductor film to be formed.

In order to obtain a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film, besides the high vacuum evacuation of the chamber, a high purification of a sputtering gas is also needed. As an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be prevented as much as possible.

Here, a 35-nm-thick In—Ga—Zn oxide film is formed as the oxide semiconductor film by a sputtering method using an In—Ga—Zn oxide target (In:Ga:Zn=1:1:1).

Then, after a mask is formed over the oxide semiconductor film 18 by a photolithography process using a second photomask, the oxide semiconductor film is partly etched using the mask. Thus, the oxide semiconductor film 19 a and an oxide semiconductor film 19 c subjected to element isolation as illustrated in FIG. 6D are formed. After that, the mask is removed.

Here, the oxide semiconductor films 19 a and 19 c are formed in such a manner that a mask is formed over the oxide semiconductor film 18 and part of the oxide semiconductor film 18 is selectively etched by a wet etching method.

Next, as illustrated in FIG. 7A, a conductive film 20 to be the conductive films 21 a and 21 b later is formed.

The conductive film 20 can be formed by a method similar to that of the conductive film 12 as appropriate.

Here, a 50-nm-thick tungsten film and a 300-nm-thick copper film are sequentially stacked by a sputtering method.

Next, a mask is formed over the conductive film 20 by a photolithography process using a third photomask. Then, the conductive film 20 is etched using the mask, so that the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode are formed as illustrated in FIG. 7B. After that, the mask is removed.

Here, a mask is formed over the copper film by a photolithography process. Then, the tungsten film and the copper film are etched with the use of the mask, so that the conductive films 21 a and 21 b are formed. Note that the copper film is etched by a wet etching method. Next, the tungsten film is etched by a dry etching method using SF₆, whereby fluoride is formed on the surface of the copper film. By the fluoride, diffusion of copper elements from the copper film is reduced and thus the copper concentration in the oxide semiconductor film 19 a can be reduced.

Next, as illustrated in FIG. 7C, an oxide insulating film 22 to be the oxide insulating film 23 later and an oxide insulating film 24 to be the oxide insulating film 25 later are formed over the oxide semiconductor films 19 a and 19 c and the conductive films 21 a and 21 b. The oxide insulating film 22 and the oxide insulating film 24 can each be formed by a method similar to those of the nitride insulating film 15 and the oxide insulating film 16 as appropriate.

Note that after the oxide insulating film 22 is formed, the oxide insulating film 24 is preferably formed in succession without exposure to the air. After the oxide insulating film 22 is formed, the oxide insulating film 24 is formed in succession by adjusting at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature without exposure to the air, whereby the concentration of impurities attributed to the atmospheric component at the interface between the oxide insulating film 22 and the oxide insulating film 24 can be reduced and oxygen in the oxide insulating film 24 can be moved to the oxide semiconductor film 19 a; accordingly, the amount of oxygen vacancies in the oxide semiconductor film 19 a can be reduced.

As the oxide insulating film 22, a silicon oxide film or a silicon oxynitride film can be formed under the following conditions: the substrate placed in a treatment chamber of a plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 280° C. and lower than or equal to 400° C., the pressure is greater than or equal to 20 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 250 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power is supplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferably used as the source gas of the oxide insulating film 22. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given as examples.

With the use of the above conditions, an oxide insulating film which is permeated by oxygen can be formed as the oxide insulating film 22. Further, by providing the oxide insulating film 22, damage to the oxide semiconductor film 19 a can be reduced in a step of forming the oxide insulating film 25 which is formed later.

Under the above film formation conditions, the bonding strength of silicon and oxygen becomes strong in the above substrate temperature range. Thus, as the oxide insulating film 22, a dense and hard oxide insulating film which is permeated by oxygen, typically, a silicon oxide film or a silicon oxynitride film having an etching rate lower than or equal to 10 nm/min, preferably lower than or equal to 8 nm/min when etching is performed at 25° C. using hydrofluoric acid of 0.5 wt % can be formed.

The oxide insulating film 22 is formed while heating is performed; thus, hydrogen, water, or the like contained in the oxide semiconductor film 19 a can be released in the step. Hydrogen contained in the oxide semiconductor film 19 a is bonded to an oxygen radical formed in plasma to form water. Since the substrate is heated in the step of forming the oxide insulating film 22, water formed by bonding of oxygen and hydrogen is released from the oxide semiconductor film. That is, when the oxide insulating film 22 is formed by a plasma CVD method, the amount of water and hydrogen contained in the oxide semiconductor film 19 a can be reduced.

Further, time for heating in a state where the oxide semiconductor film 19 a is exposed can be shortened because heating is performed in a step of forming the oxide insulating film 22. Thus, the amount of oxygen released from the oxide semiconductor film by heat treatment can be reduced. That is, the amount of oxygen vacancies in the oxide semiconductor film can be reduced.

Note that when the ratio of the amount of the oxidizing gas to the amount of the deposition gas containing silicon is 100 or higher, the hydrogen content in the oxide insulating film 22 can be reduced. Consequently, the amount of hydrogen entering the oxide semiconductor film 19 a can be reduced; thus, the negative shift in the threshold voltage of the transistor can be inhibited.

Here, as the oxide insulating film 22, a 50-nm-thick silicon oxynitride film is formed by a plasma CVD method in which silane with a flow rate of 30 sccm and dinitrogen monoxide with a flow rate of 4000 sccm are used as a source gas, the pressure in the treatment chamber is 200 Pa, the substrate temperature is 220° C., and a high-frequency power of 150 W is supplied to parallel-plate electrodes with the use of a 27.12 MHz high-frequency power source. Under the above conditions, a silicon oxynitride film which is permeated by oxygen can be formed.

As the oxide insulating film 24, a silicon oxide film or a silicon oxynitride film is formed under the following conditions: the substrate placed in a treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., preferably higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², preferably greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

A deposition gas containing silicon and an oxidizing gas are preferably used as the source gas of the oxide insulating film 24. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given as examples.

As the film formation conditions of the oxide insulating film 24, the high-frequency power having the above power density is supplied to the treatment chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; therefore, the oxygen content in the oxide insulating film 24 becomes higher than that in the stoichiometric composition. On the other hand, in the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in a later step. Thus, it is possible to form an oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition and from which part of oxygen is released by heating. Further, the oxide insulating film 22 is provided over the oxide semiconductor film 19 a. Accordingly, in the step of forming the oxide insulating film 24, the oxide insulating film 22 functions as a protective film of the oxide semiconductor film 19 a. Consequently, the oxide insulating film 24 can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film 19 a is reduced.

Here, as the oxide insulating film 24, a 400-nm-thick silicon oxynitride film is formed by a plasma CVD method in which silane with a flow rate of 200 sccm and dinitrogen monoxide with a flow rate of 4000 sccm are used as the source gas, the pressure in the treatment chamber is 200 Pa, the substrate temperature is 220° C., and a high-frequency power of 1500 W is supplied to the parallel-plate electrodes with the use of a 27.12 MHz high-frequency power source. Note that the plasma CVD apparatus is a parallel-plate plasma CVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 0.25 W/cm².

Further, when the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode is formed, the oxide semiconductor film 19 a is damaged by the etching of the conductive film, so that oxygen vacancies are generated on the back channel side of the oxide semiconductor film 19 a (the side of the oxide semiconductor film 19 a which is opposite to the side facing to the conductive film 13 functioning as a gate electrode). However, with the use of the oxide insulating film which contains oxygen at a higher proportion than the stoichiometric composition as the oxide insulating film 24, the oxygen vacancies generated on the back channel side can be repaired by heat treatment. By this, defects contained in the oxide semiconductor film 19 a can be reduced, and thus, the reliability of the transistor 102 can be improved.

Then, a mask is formed over the oxide insulating film 24 by a photolithography process using a fourth photomask. Next, as illustrated in FIG. 7D, part of the oxide insulating film 22 and part of the oxide insulating film 24 are etched with the use of the mask to form the oxide insulating film 23 and the oxide insulating film 25 having the opening portion 40. After that, the mask is removed.

In the process, the oxide insulating films 22 and 24 are preferably etched by a dry etching method. As a result, the oxide semiconductor film 19 c is exposed to plasma in the etching treatment; thus, the amount of oxygen vacancies in the oxide semiconductor film 19 c can be increased.

Next, heat treatment is performed. The heat treatment is performed typically at a temperature of higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C.

An electric furnace, an RTA apparatus, or the like can be used for the heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature of higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened.

The heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas (argon, helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like.

By the heat treatment, part of oxygen contained in the oxide insulating film 25 can be moved to the oxide semiconductor film 19 a, so that the amount of oxygen vacancies contained in the oxide semiconductor film 19 a can be reduced.

In the case where water, hydrogen, or the like is contained in the oxide insulating film 23 and the oxide insulating film 25 and the nitride insulating film 26 has a barrier property against water, hydrogen, or the like, when the nitride insulating film 26 is formed later and heat treatment is performed, water, hydrogen, or the like contained in the oxide insulating film 23 and the oxide insulating film 25 are moved to the oxide semiconductor film 19 a, so that defects are generated in the oxide semiconductor film 19 a. However, by the heating, water, hydrogen, or the like contained in the oxide insulating film 23 and the oxide insulating film 25 can be released; thus, variation in electrical characteristics of the transistor 102 can be reduced, and change in threshold voltage can be inhibited.

Note that when the oxide insulating film 24 is formed over the oxide insulating film 22 while being heated, oxygen can be moved to the oxide semiconductor film 19 a to reduce the amount of oxygen vacancies in the oxide semiconductor film 19 a; thus, the heat treatment is not necessarily performed.

The heat treatment may be performed after the formation of the oxide insulating films 22 and 24. However, the heat treatment is preferably performed after the formation of the oxide insulating films 23 and 25 because a film having higher conductivity can be formed in such a manner that oxygen is not moved to the oxide semiconductor film 19 c and oxygen is released from the oxide semiconductor film 19 c because of exposure of the oxide semiconductor film 19 c and then oxygen vacancies are generated.

Here, the heat treatment is performed at 350° C. in an atmosphere of nitrogen and oxygen for one hour.

Then, as illustrated in FIG. 8A, the nitride insulating film 26 is formed.

The nitride insulating film 26 can be formed by a method similar to those of the nitride insulating film 15 and the oxide insulating film 16 as appropriate. By forming the nitride insulating film 26 by a sputtering method, a CVD method, or the like, the oxide semiconductor film 19 c is exposed to plasma; thus, the amount of oxygen vacancies in the oxide semiconductor film 19 c can be increased.

The oxide semiconductor film 19 c has improved conductivity and functions as the pixel electrode 19 b. When a silicon nitride film is formed by a plasma CVD method as the nitride insulating film 26, hydrogen contained in the silicon nitride film is diffused into the oxide semiconductor film 19 c; thus, the conductivity of the pixel electrode 19 b can be enhanced.

In the case where a silicon nitride film is formed by a plasma CVD method as the nitride insulating film 26, the substrate placed in the treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is preferably held at a temperature higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C., so that a dense silicon nitride film can be formed.

In the case where a silicon nitride film is formed, a deposition gas containing silicon, nitrogen, and ammonia are preferably used as a source gas. As the source gas, a small amount of ammonia compared to the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species cleave a bond between silicon and hydrogen which are contained in a deposition gas containing silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia is larger than the amount of nitrogen in the source gas, cleavage of a deposition gas containing silicon and cleavage of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, the flow ratio of the nitrogen to the ammonia is set to be preferably greater than or equal to 5 and less than or equal to 50, more preferably greater than or equal to 10 and less than or equal to 50.

Here, in the treatment chamber of a plasma CVD apparatus, a 50-nm-thick silicon nitride film is formed as the nitride insulating film 26 by a plasma CVD method in which silane with a flow rate of 50 sccm, nitrogen with a flow rate of 5000 sccm, and ammonia with a flow rate of 100 sccm are used as the source gas, the pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and a high-frequency power of 1000 W is supplied to parallel-plate electrodes with a high-frequency power supply of 27.12 MHz. Note that the plasma CVD apparatus is a parallel-plate plasma CVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 1.7×10⁻¹ W/cm².

Next, heat treatment may be performed. The heat treatment is performed typically at a temperature of higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C. As a result, the negative shift of the threshold voltage can be reduced. Moreover, the amount of change in the threshold voltage can be reduced.

Next, although not illustrated, a mask is formed over the nitride insulating film 26 by a photolithography process using a fifth photomask, and the nitride insulating film 26 is etched using the mask. Thus, a conductive film formed at the same time as the conductive films 21 a and 21 b is exposed and the nitride insulating film 27 is formed. The conductive film is connected to the common electrode 29 to be formed later.

Next, as illustrated in FIG. 8B, a conductive film 28 to be the common electrode 29 later is formed over the nitride insulating film 27.

The conductive film 28 is formed by a sputtering method, a CVD method, an evaporation method, or the like.

Then, a mask is formed over the conductive film 28 by a photolithography process using a sixth photomask. Next, as illustrated in FIG. 8C, part of the conductive film 28 is etched with the use of the mask to form the common electrode 29. Although not illustrated, the common electrode 29 is connected to a connection terminal formed at the same time as the conductive film 13 or a connection terminal formed at the same time as the conductive films 21 a and 21 b. After that, the mask is removed.

Through the above process, the transistor 102 is manufactured and the capacitor 105 can also be manufactured.

The element substrate of the display device of this embodiment is provided with a common electrode including stripe regions extending in a direction intersecting with a signal line. Therefore, the display device can have excellent contrast.

On the element substrate of the display device of this embodiment, the pixel electrode is formed at the same time as the oxide semiconductor film of the transistor; therefore, the transistor 102 and the capacitor 105 can be formed using six photomasks. The pixel electrode functions as the one of electrodes of the capacitor. The common electrode functions as the other of electrodes of the capacitor. Thus, a step of forming another conductive film is not needed to form the capacitor, and the number of steps of manufacturing the display device can be reduced. The capacitor has a light-transmitting property. As a result, the area occupied by the capacitor can be increased and the aperture ratio in a pixel can be increased.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Modification Example 1

A structure in which a common line connected to the common electrode is provided in the display device described in Embodiment 1 is described with reference to FIGS. 9A and 9B.

FIG. 9A is a top view illustrating the pixels 103 a, 103 b, and 103 c included in a display device, and FIG. 9B is a cross-sectional view taken along dashed-dotted lines A-B and C-D in FIG. 9A.

As illustrated in FIG. 9A, a common line 21 c extending in a direction parallel or substantially parallel to the conductive film 21 a functioning as a signal line is formed. For easy understanding of the structure of the common electrode 29, the common electrode 29 is hatched in FIG. 9A to explain its shape. The common electrode 29 includes a plurality of first regions hatched with diagonally left down lines and a second region hatched with diagonally right down lines. The plurality of the first regions is a plurality of stripe regions. The second region extends in a direction parallel or substantially parallel to the conductive film 21 a functioning as a signal line. The second region can be referred to as a connection region connecting to the plurality of first regions (stripe regions). The common line 21 c overlaps with the connection region (second region) of the common electrode 29.

The common line 21 c may be provided par pixel. Alternatively, the common line 21 c may be provided every plurality of pixels. For example, as illustrated in FIG. 9A, one common line 21 c is provided for every three pixels, so that the area occupied by the common line in the display device can be reduced. As a result, the area of the pixel and the aperture ratio of the pixel can be increased.

In a region where the pixel electrode 19 b and the common electrode 29 overlap with each other, a liquid crystal molecule is less likely to be driven by an electric field generated between the pixel electrode 19 b and the connection region (second region) of the common electrode 29. Therefore, the area of a region overlapping with the pixel electrode 19 b in the connection region of the common electrode 29 is reduced, so that a region where a liquid crystal molecule is driven can be increased, leading to an increase in the aperture ratio. For example, as illustrated in FIG. 9A, the connection region of the common electrode 29 is provided so as not to overlap with the pixel electrode 19 b, whereby the area of a region where the pixel electrode 19 b and the common electrode 29 overlap with each other can be reduced and thus the aperture ratio of the pixel can be increased.

Although one common line 21 c is provided for the three pixels 103 a, 103 b, and 103 c in FIG. 9A, one common line may be provided for every two pixels. Alternatively, one common line may be provided for every four or more pixels.

As illustrated in FIG. 9B, the common line 21 c can be formed at the same time as the conductive film 21 a functioning as a signal line. The common electrode 29 is connected to the common line 21 c in an opening portion 42 formed in the oxide insulating film 23, the oxide insulating film 25, and the nitride insulating film 27.

Since a material of the conductive film 21 a has resistivity lower than that of the common electrode 29, resistance of the common electrode 29 and the common line 21 c can be reduced.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, a display device which is different from the display device in Embodiment 2 and a manufacturing method thereof are described with reference to a drawing. This embodiment is different from Embodiment 2 in that a transistor included in a high resolution display device includes a source electrode and a drain electrode capable of reducing light leakage. Note that the description of the same structures as those in Embodiment 2 is omitted.

FIG. 10 is a top view of a display device described in this embodiment. The conductive film 21 b functioning as one of a source electrode and a drain electrode has an L shape in the top view. In other words, the conductive film 21 b has a shape in which a region 21 b_1 extending in a direction perpendicular to the conductive film 13 functioning as a scan line and a region 21 b_2 extending in a direction parallel or substantially parallel to the conductive film 13 are connected to each other in the top view. The region 21 b_2 overlaps with one or more of the conductive film 13, the pixel electrode 19 b, and the common electrode 29 in the top view. Alternatively, the conductive film 21 b includes the region 21 b_2 extending in a direction parallel or substantially parallel to the conductive film 13 and the region 21 b_2 is placed between the conductive film 13 and the pixel electrode 19 b or the common electrode 29 in the top view.

Since the area of the pixel in a high resolution display device is reduced, the distance between the common electrode 29 and the conductive film 13 functioning as a scan line is reduced. In a pixel performing black display, when voltage at which a transistor is turned on is applied to the conductive film 13 functioning as a scan line, an electric field is generated between the pixel electrode 19 b and the conductive film 13 functioning as a scan line. As a result, a liquid crystal molecule rotates in an unintended direction, causing light leakage.

However, in the transistor included in the display device of this embodiment, the conductive film 21 b functioning as the one of a source electrode and a drain electrode includes the region 21 b_2 overlapping with one or more of the conductive film 13, the pixel electrode 19 b, and the common electrode 29, or the region 21 b_2 placed between the conductive film 13 and the pixel electrode 19 b or the common electrode 29 in the top view. As a result, the region 21 b_2 blocks the electric field of the conductive film 13 functioning as a scan line and an electric field generated between the conductive film 13 and the pixel electrode 19 b can be suppressed, leading to a reduction in light leakage.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, a display device which is different from the display devices in Embodiments 2 and 3 and a manufacturing method thereof are described with reference to drawings. This embodiment is different from Embodiment 2 in that a high resolution display device includes a common electrode capable of reducing light leakage. Note that the description of the same structures as those in Embodiment 2 is omitted.

FIG. 11 is a top view of a display device described in this embodiment. A common electrode 29 a includes stripe regions 29 a_1 extending in a direction intersecting with the conductive film 21 a functioning as a signal line and a region 29 a_2 which is connected to the stripe regions and overlaps with the conductive film 13 functioning as a scan line.

Since the area of a pixel is reduced in a high resolution display device, the distance between the pixel electrode 19 b and the conductive film 13 functioning as a scan line is reduced. When voltage is applied to the conductive film 13 functioning as a scan line, an electric field is generated between the conductive film 13 and the pixel electrode 19 b. As a result, a liquid crystal molecule rotates in an unintended direction, causing light leakage.

However, the display device described in this embodiment includes the common electrode 29 a including the region 29 a_2 intersecting with the conductive film 13 functioning as a scan line. Therefore, an electric field can be prevented from being generated between the common electrode 29 a and the conductive film 13 functioning as a scan line, leading to a reduction in light leakage.

Note that a top view of one embodiment of the present invention is not limited to FIG. 11. The display device can have a variety of different structures. For example, as illustrated in FIG. 24 or FIG. 25, the common electrode 29 a may include a region overlapping with part of the conductive film 13 functioning as a scan line. A channel region formed in the oxide semiconductor film 19 a of the transistor does not overlap with the common electrode 29 a. Accordingly, an electric field of the common electrode 29 a is not applied to the channel region, resulting in a reduction in leakage current of the transistor. Furthermore, the common electrode 29 a in FIG. 25 includes a region overlapping with the conductive film 13 functioning as a scan line and the conductive film 21 a functioning as a signal line. Therefore, electric fields of the conductive film 13 and the conductive film 21 a can be blocked by the common electrode 29 a, so that alignment disorder of liquid crystal molecules can be reduced.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 5

In this embodiment, a display device which is different from the display device in Embodiment 2 and a manufacturing method thereof are described with reference to drawings. The display device in this embodiment is different from that in Embodiment 2 in that the transistor has a structure in which an oxide semiconductor film is provided between different gate electrodes, that is, a dual-gate structure. Note that the description of the same structures as those in Embodiment 2 is omitted.

A specific structure of an element substrate included in the display device is described. The element substrate in this embodiment is different from that in Embodiment 2 in that a conductive film 29 b functioning as a gate electrode and overlapping part of or the whole of each of the conductive film 13 functioning as a gate electrode, the oxide semiconductor film 19 a, the conductive films 21 a and 21 b, and the oxide insulating film 25 is provided as shown in FIG. 12. The conductive film 29 b functioning as a gate electrode is connected to the conductive film 13 functioning as a gate electrode in opening portions 41 a and 41 b.

A transistor 102 a shown in FIG. 12 is a channel-etched transistor. Note that a cross-sectional view along line A-B shows the transistor 102 a in the channel length direction and a capacitor 105 a, and a cross-sectional view along line C-D shows the transistor 102 a in the channel width direction and a connection portion between the conductive film 13 functioning as a gate electrode and the conductive film 29 b functioning as a gate electrode.

The transistor 102 a in FIG. 12 has a dual-gate structure and includes the conductive film 13 functioning as a gate electrode over the substrate 11. In addition, the transistor 102 a includes the nitride insulating film 15 formed over the substrate 11 and the conductive film 13 functioning as a gate electrode, the oxide insulating film 17 formed over the nitride insulating film 15, the oxide semiconductor film 19 a overlapping with the conductive film 13 functioning as a gate electrode with the nitride insulating film 15 and the oxide insulating film 17 provided therebetween, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode which are in contact with the oxide semiconductor film 19 a. Moreover, the oxide insulating film 23 is formed over the oxide insulating film 17, the oxide semiconductor film 19 a, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, and the oxide insulating film 25 is formed over the oxide insulating film 23. The nitride insulating film 27 is formed over the nitride insulating film 15, the oxide insulating film 23, the oxide insulating film 25, and the conductive film 21 b. The pixel electrode 19 b is formed over the oxide insulating film 17. The pixel electrode 19 b is connected to one of the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, here, connected to the conductive film 21 b. The common electrode 29 and the conductive film 29 b functioning as a gate electrode are formed over the nitride insulating film 27.

As illustrated in the cross-sectional view along line C-D, the conductive film 29 b functioning as a gate electrode is connected to the conductive film 13 functioning as a gate electrode in the opening portions 41 a and 41 b provided in the nitride insulating film 15 and the nitride insulating film 27. That is, the conductive film 13 functioning as a gate electrode and the conductive film 29 b functioning as a gate electrode have the same potential.

Thus, by applying voltage at the same potential to each gate electrode of the transistor 102 a, variation in the initial characteristics can be reduced, and degradation of the transistor 102 a after the −GBT stress test and a change in the rising voltage of on-state current at different drain voltages can be suppressed. In addition, a region where carriers flow in the oxide semiconductor film 19 a is increased in the film thickness direction, so that the amount of transferred carriers is increased. As a result, the on-state current of the transistor 102 a is increased, and the field-effect mobility is increased. Typically, the field-effect mobility is greater than or equal to 20 cm²/V·s.

Over the transistor 102 a in this embodiment, the oxide insulating films 23 and 25, which are subjected to element isolation, are formed. The oxide insulating films 23 and 25 overlap with the oxide semiconductor film 19 a. In the cross-sectional view in the channel width direction, end portions of the oxide insulating films 23 and 25 are located on an outer side of the oxide semiconductor film 19 a. Furthermore, in the channel width direction in FIG. 12, the conductive film 29 b functioning as a gate electrode faces a side surface of the oxide semiconductor film 19 a with the oxide insulating films 23 and 25 provided therebetween.

An end portion processed by etching or the like of the oxide semiconductor film, in which defects are generated by damage due to processing, is also contaminated by the attachment of an impurity, or the like. Thus, the end portion of the oxide semiconductor film is easily activated by application of a stress such as an electric field, thereby easily becoming n-type (having a low resistance). Therefore, the end portion of the oxide semiconductor film 19 a overlapping with the conductive film 13 functioning as a gate electrode easily becomes n-type. When the end portion which becomes n-type is provided between the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, the region which becomes n-type serves as a carrier path, resulting in a parasitic channel. However, as illustrated in the cross-sectional view along line C-D, when the conductive film 29 b functioning as a gate electrode faces a side surface of the oxide semiconductor film 19 a with the oxide insulating films 23 and 25 provided therebetween in the channel width direction, due to the electric field of the conductive film 29 b functioning as a gate electrode, generation of a parasitic channel on the side surface of the oxide semiconductor film 19 a or in a region including the side surface and the vicinity of the side surface is suppressed. As a result, a transistor which has excellent electrical characteristics such as a sharp increase in the drain current at the threshold voltage is obtained.

The common electrode includes the stripe regions extending in a direction intersecting with a signal line. Accordingly, in the vicinity of the pixel electrode 19 b and the conductive film 21 a, unintended alignment of liquid crystal molecules can be prevented and thus light leakage can be suppressed. As a result, a display device with excellent contrast can be manufactured.

In the capacitor 105 a, the pixel electrode 19 b is formed at the same time as the oxide semiconductor film 19 a and has increased conductivity by containing an impurity. Alternatively, the pixel electrode 19 b is formed at the same time as the oxide semiconductor film 19 a, and has increased conductivity by containing oxygen vacancies generated by plasma damage or the like. Alternatively, the pixel electrode 19 b is formed at the same time as the oxide semiconductor film 19 a, and has increased conductivity by containing impurities and oxygen vacancies generated by plasma damage or the like.

On the element substrate of the display device in this embodiment, the pixel electrode is formed at the same time as the oxide semiconductor film of the transistor. The pixel electrode functions as one of electrodes of the capacitor. The common electrode functions as the other of electrodes of the capacitor. Thus, a step of forming another conductive film is not needed to form the capacitor, and the number of steps of manufacturing the display device can be reduced. The capacitor has a light-transmitting property. As a result, the area occupied by the capacitor can be increased and the aperture ratio in a pixel can be increased.

Details of the transistor 102 a are described below. Note that the description of the components with the same reference numerals as those in Embodiment 2 is omitted.

The conductive film 29 b functioning as a gate electrode can be formed using a material similar to that of the common electrode 29 in Embodiment 2.

Next, a method for manufacturing the transistor 102 a and the capacitor 105 a in FIG. 12 is described with reference to FIGS. 6A to 6D, FIGS. 7A to 7D, FIG. 8A, and FIGS. 13A to 13C.

As in Embodiment 2, through the steps illustrated in FIGS. 6A to 6D, FIGS. 7A to 7D, and FIG. 8A, the conductive film 13 functioning as a gate electrode, the nitride insulating film 15, the oxide insulating film 16, the oxide semiconductor film 19 a, the pixel electrode 19 b, the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, the oxide insulating film 22, the oxide insulating film 24, and the nitride insulating film 26 are formed over the substrate 11. In these steps, the photography processes using the first to fourth photomasks are performed.

Next, a mask is formed over the nitride insulating film 26 through a photolithography process using a fifth photomask, and then part of the nitride insulating film 26 is etched using the mask; thus, the nitride insulating film 27 having the opening portions 41 a and 41 b is formed as illustrated in FIG. 13A.

Next, as illustrated in FIG. 13B, the conductive film 28 to be the common electrode 29 and the conductive film 29 b functioning as a gate electrode is formed over the conductive film 13 functioning as a gate electrode, the conductive film 21 b, and the nitride insulating film 27.

Then, a mask is formed over the conductive film 28 by a photolithography process using a sixth photomask. Next, as illustrated in FIG. 13C, part of the conductive film 28 is etched with the use of the mask to form the common electrode 29 and the conductive film 29 b functioning as a gate electrode. After that, the mask is removed.

Through the above process, the transistor 102 a is manufactured and the capacitor 105 a can also be manufactured.

In the transistor described in this embodiment, since the common electrode 29 functioning as a gate electrode faces a side surface of the oxide semiconductor film 19 a with the oxide insulating films 23 and 25 provided therebetween in the channel width direction, due to the electric field of the conductive film 29 b functioning as a gate electrode, generation of a parasitic channel on the side surface of the oxide semiconductor film 19 a or in a region including the side surface and the vicinity of the side surface is suppressed. As a result, a transistor which has excellent electrical characteristics such as a sharp increase in the drain current at the threshold voltage is obtained.

The element substrate of the display device of this embodiment is provided with a common electrode including stripe regions extending in a direction intersecting with a signal line. Therefore, the display device can have excellent contrast.

On the element substrate of the display device in this embodiment, the pixel electrode is formed at the same time as the oxide semiconductor film of the transistor. The pixel electrode functions as one of electrodes of the capacitor. The common electrode functions as the other of electrodes of the capacitor. Thus, a step of forming another conductive film is not needed to form the capacitor, and the number of steps of manufacturing the display device can be reduced. The capacitor has a light-transmitting property. As a result, the area occupied by the capacitor can be increased and the aperture ratio in a pixel can be increased.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 6

As for the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode provided in each of the transistors described in Embodiments 2 to 5, it is possible to use a conductive material which is easily bonded to oxygen, such as tungsten, titanium, aluminum, copper, molybdenum, chromium, or tantalum, or an alloy thereof. Thus, oxygen contained in the oxide semiconductor film 19 a and the conductive material contained in the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode are bonded to each other, so that an oxygen vacancy region is formed in the oxide semiconductor film 19 a. Further, in some cases, part of constituent elements of the conductive material that forms the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode is mixed into the oxide semiconductor film 19 a. Consequently, low-resistance regions are formed in the vicinity of regions of the oxide semiconductor film 19 a which are in contact with the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode. The low-resistance regions are formed between the oxide insulating film 17 and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode so as to be in contact with the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode. Since the low-resistance regions have high conductivity, contact resistance between the oxide semiconductor film 19 a and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode can be reduced, and thus, the on-state current of the transistor can be increased.

Further, the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode may each have a stacked-layer structure of the conductive material which is easily bonded to oxygen and a conductive material which is not easily bonded to oxygen, such as titanium nitride, tantalum nitride, or ruthenium. With such a stacked-layer structure, oxidization of the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode can be prevented at the interface between the oxide insulating film 23 and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode, so that an increase in the resistance of the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode can be inhibited.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 7

In this embodiment, a display device including a transistor in which the amount of defects in an oxide semiconductor film can be further reduced as compared to Embodiments 2 to 5 is described with reference to drawings. The transistor described in this embodiment is different from any of the transistors in Embodiments 2 to 5 in that a multilayer film including a plurality of oxide semiconductor films is provided. Here, details are described using the transistor in Embodiment 2.

FIGS. 14A and 14B each illustrate a cross-sectional view of an element substrate included in a display device. FIGS. 14A and 14B are cross-sectional views taken along lines A-B and C-D in FIG. 4.

A transistor 102 b in FIG. 14A includes a multilayer film 37 a overlapping with the conductive film 13 functioning as a gate electrode with the nitride insulating film 15 and the oxide insulating film 17 provided therebetween, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode in contact with the multilayer film 37 a. The oxide insulating film 23, the oxide insulating film 25, and the nitride insulating film 27 are formed over the nitride insulating film 15, the oxide insulating film 17, the multilayer film 37 a, and the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode.

The capacitor 105 b in FIG. 14A includes a multilayer film 37 b formed over the oxide insulating film 17, the nitride insulating film 27 in contact with the multilayer film 37 b, and the common electrode 29 in contact with the nitride insulating film 27. The multilayer film 37 b includes an oxide semiconductor film 19 f and an oxide semiconductor film 39 b. In other words, the multilayer film 37 b has a two-layer structure. The multilayer film 37 b functions as a pixel electrode.

In the transistor 102 b described in this embodiment, the multilayer film 37 a includes the oxide semiconductor film 19 a and an oxide semiconductor film 39 a. That is, the multilayer film 37 a has a two-layer structure. Part of the oxide semiconductor film 19 a serves as a channel region. Furthermore, the oxide insulating film 23 is formed in contact with the oxide semiconductor film 39 a, and the oxide insulating film 25 is formed in contact with the oxide insulating film 23. That is, the oxide semiconductor film 39 a is provided between the oxide semiconductor film 19 a and the oxide insulating film 23.

The oxide semiconductor film 39 a is an oxide film containing one or more elements that form the oxide semiconductor film 19 a. Thus, interface scattering is unlikely to occur at the interface between the oxide semiconductor films 19 a and 39 a. Accordingly, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interface.

The oxide semiconductor film 39 a is typically an In—Ga oxide film, an In—Zn oxide film, or an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd). The energy at the conduction band bottom of the oxide semiconductor film 39 a is closer to a vacuum level than that of the oxide semiconductor film 19 a is, and typically, the difference between the energy at the conduction band bottom of the oxide semiconductor film 39 a and the energy at the conduction band bottom of the oxide semiconductor film 19 a is any one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. That is, the difference between the electron affinity of the oxide semiconductor film 39 a and the electron affinity of the oxide semiconductor film 19 a is any one of 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and any one of 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

The oxide semiconductor film 39 a preferably contains In because carrier mobility (electron mobility) can be increased.

When the oxide semiconductor film 39 a contains a larger amount of Al, Ga, Y, Zr, La, Ce, or Nd in an atomic ratio than the amount of In in an atomic ratio, any of the following effects may be obtained: (1) the energy gap of the oxide semiconductor film 39 a is widened; (2) the electron affinity of the oxide semiconductor film 39 a is reduced; (3) scattering of impurities from the outside is reduced; (4) an insulating property increases as compared to the oxide semiconductor films 19 a; and (5) oxygen vacancies are less likely to be generated because Al, Ga, Y, Zr, La, Ce, and Nd are metal elements which are strongly bonded to oxygen.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxide film, the proportions of In and M when summation of In and M is assumed to be 100 atomic % are preferably as follows: the atomic percentage of In is less than 50 atomic % and the atomic percentage of M is greater than or equal to 50 atomic %, or more preferably, the atomic percentage of In is less than 25 atomic % and the atomic percentage of M is greater than or equal to 75 atomic %.

Furthermore, in the case where each of the oxide semiconductor films 19 a and 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd), the proportion of M atoms (M represents Al, Ga, Y, Zr, La, Ce, or Nd) in the oxide semiconductor film 39 a is higher than that in the oxide semiconductor film 19 a. As a typical example, the proportion of M in the oxide semiconductor film 39 a is 1.5 or more times, preferably twice or more, further preferably three or more times as high as that in the oxide semiconductor film 19 a.

Furthermore, in the case where each of the oxide semiconductor films 19 a and 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd), when In:M:Zn=x₁:y₁:z₁ [atomic ratio] is satisfied in the oxide semiconductor film 39 a and In:M:Zn=x₂:y₂:z₂ [atomic ratio] is satisfied in the oxide semiconductor film 19 a, y₁/x₁ is higher than y₂/x₂, and y₁/x₁ is preferably 1.5 or more times, more preferably twice or more, still more preferably three or more time as high as y₂/x₂.

In the case where the oxide semiconductor film 19 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for forming the oxide semiconductor film 19 a, x₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6, and z₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₁/y₁ is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film as the oxide semiconductor film 19 a is easily formed. Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2.

In the case where the oxide semiconductor film 39 a is an In-M-Zn oxide film (M represents Al, Ga, Y, Zr, La, Ce, or Nd) and a target having the atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for forming the oxide semiconductor film 39 a, x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₂/y₂ is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film as the oxide semiconductor film 39 a is easily formed. Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:6:8.

Note that the proportion of each metal element in the atomic ratio of each of the oxide semiconductor films 19 a and the oxide semiconductor film 39 a varies within a range of ±40% of that in the above atomic ratio as an error.

The oxide semiconductor film 39 a also functions as a film that relieves damage to the oxide semiconductor film 19 a at the time of forming the oxide insulating film 25 later.

The thickness of the oxide semiconductor film 39 a is greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor film 39 a may have a non-single-crystal structure, for example, like the oxide semiconductor film 19 a. The non-single-crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure which is described later, or an amorphous structure, for example.

The oxide semiconductor film 39 a may have an amorphous structure, for example. The oxide semiconductor films having the amorphous structure each have disordered atomic arrangement and no crystalline component, for example.

Note that the oxide semiconductor films 19 a and 39 a may each be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure. The mixed film has a single-layer structure including, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Further, in some cases, the mixed film has a stacked-layer structure in which two or more of the following regions are stacked: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure.

Here, the oxide semiconductor film 39 a is provided between the oxide semiconductor film 19 a and the oxide insulating film 23. Thus, if carrier traps are formed between the oxide semiconductor film 39 a and the oxide insulating film 23 by impurities and defects, electrons flowing in the oxide semiconductor film 19 a are less likely to be captured by the carrier traps because there is a distance between the carrier traps and the oxide semiconductor film 19 a. Accordingly, the amount of on-state current of the transistor can be increased, and the field-effect mobility can be increased. When the electrons are captured by the carrier traps, the electrons become negative fixed charges. As a result, a threshold voltage of the transistor fluctuates. However, by the distance between the oxide semiconductor film 19 a and the carrier traps, capture of electrons by the carrier traps can be reduced, and accordingly, fluctuations of the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductor film 39 a, and accordingly, the amount of impurities that are transferred from the outside to the oxide semiconductor film 19 a can be reduced. Furthermore, an oxygen vacancy is less likely to be formed in the oxide semiconductor film 39 a. Consequently, the impurity concentration and the amount of oxygen vacancies in the oxide semiconductor film 19 a can be reduced.

Note that the oxide semiconductor films 19 a and 39 a are not formed by simply stacking each film, but are formed to form a continuous junction (here, in particular, a structure in which the energy of the conduction band bottom is changed continuously between each film). In other words, a stacked-layer structure in which there exists no impurity that forms a defect state such as a trap center or a recombination center at each interface is provided. If an impurity exists between the oxide semiconductor films 19 a and 39 a that are stacked, a continuity of the energy band is damaged, and the carrier is captured or recombined at the interface and then disappears.

In order to form such a continuous junction, it is necessary to form films continuously without being exposed to air, with the use of a multi-chamber deposition apparatus (sputtering apparatus) including a load lock chamber. Each chamber in the sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity against the oxide semiconductor film, as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas containing carbon or hydrogen from an exhaust system to the inside of the chamber.

As in a transistor 102 c in FIG. 14B, a multilayer film 38 a may be provided instead of the multilayer film 37 a.

In addition, as in a capacitor 105 c in FIG. 14B, a multilayer film 38 b may be provided instead of the multilayer film 37 b.

The multilayer film 38 a includes an oxide semiconductor film 49 a, the oxide semiconductor film 19 a, and the oxide semiconductor film 39 a. That is, the multilayer film 38 a has a three-layer structure. Further, the oxide semiconductor film 19 a serves as a channel region.

The oxide semiconductor film 49 a can be formed using a material and a formation method similar to those of the oxide semiconductor film 39 a as appropriate.

The multilayer film 38 b includes an oxide semiconductor film 49 b, the oxide semiconductor film 19 f, and the oxide semiconductor film 39 b. In other words, the multilayer film 38 b has a three-layer structure. The multilayer film 38 b functions as a pixel electrode.

The oxide semiconductor film 19 f can be formed using a material and a formation method similar to those of the pixel electrode 19 b as appropriate. The oxide semiconductor film 49 b can be formed using a material and a formation method similar to those of the oxide semiconductor film 39 b as appropriate.

Furthermore, the oxide insulating film 17 and the oxide semiconductor film 49 a are in contact with each other. That is, the oxide semiconductor film 49 a is provided between the oxide insulating film 17 and the oxide semiconductor film 19 a.

The multilayer film 38 a and the oxide insulating film 23 are in contact with each other. In addition, the oxide semiconductor film 39 a and the oxide insulating film 23 are in contact with each other. That is, the oxide semiconductor film 39 a is provided between the oxide semiconductor film 19 a and the oxide insulating film 23.

It is preferable that the thickness of the oxide semiconductor film 49 a be smaller than that of the oxide semiconductor film 19 a. When the thickness of the oxide semiconductor film 49 a is greater than or equal to 1 nm and less than or equal to 5 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, the amount of change in the threshold voltage of the transistor can be reduced.

In the transistor described in this embodiment, the oxide semiconductor film 39 a is provided between the oxide semiconductor film 19 a and the oxide insulating film 23. Thus, if carrier traps are formed between the oxide semiconductor film 39 a and the oxide insulating film 23 by impurities and defects, electrons flowing in the oxide semiconductor film 19 a are less likely to be captured by the carrier traps because there is a distance between the carrier traps and the oxide semiconductor film 19 a. Accordingly, the amount of on-state current of the transistor can be increased, and the field-effect mobility can be increased. When the electrons are captured by the carrier traps, the electrons become negative fixed charges. As a result, a threshold voltage of the transistor changes. However, by the distance between the oxide semiconductor film 19 a and the carrier traps, capture of electrons by the carrier traps can be reduced, and accordingly, change of the threshold voltage can be reduced.

Impurities from the outside can be blocked by the oxide semiconductor film 39 a, and accordingly, the amount of impurities that are transferred from the outside to the oxide semiconductor film 19 a can be reduced. Further, an oxygen vacancy is less likely to be formed in the oxide semiconductor film 39 a. Consequently, the impurity concentration and the amount of oxygen vacancies in the oxide semiconductor film 19 a can be reduced.

Further, the oxide film 49 a is provided between the oxide insulating film 17 and the oxide semiconductor film 19 a, and the oxide semiconductor film 39 a is provided between the oxide semiconductor film 19 a and the oxide insulating film 23. Thus, it is possible to reduce the concentration of silicon or carbon in the vicinity of the interface between the oxide semiconductor film 49 a and the oxide semiconductor film 19 a, the concentration of silicon or carbon in the oxide semiconductor film 19 a, or the concentration of silicon or carbon in the vicinity of the interface between the oxide semiconductor film 39 a and the oxide semiconductor film 19 a. Consequently, in the multilayer film 38 a, the absorption coefficient derived from a constant photocurrent method is lower than 1×10⁻³/cm, preferably lower than 1×10⁻⁴/cm, and thus density of localized levels is extremely low.

The transistor 102 c having such a structure includes very few defects in the multilayer film 38 a; thus, the electrical characteristics of the transistor can be improved, and typically, the on-state current can be increased and the field-effect mobility can be improved. Furthermore, in a BT stress test and a BT photostress test which are examples of a stress test, the amount of change in threshold voltage is small, and thus, reliability is high.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 8

In this embodiment, one embodiment that can be applied to an oxide semiconductor film in the transistor included in the display device described in the above embodiment is described.

The oxide semiconductor film may include one or more of the following: an oxide semiconductor having a single-crystal structure (hereinafter referred to as a single-crystal oxide semiconductor); an oxide semiconductor having a polycrystalline structure (hereinafter referred to as a polycrystalline oxide semiconductor); an oxide semiconductor having a microcrystalline structure (hereinafter referred to as a microcrystalline oxide semiconductor); and an oxide semiconductor having an amorphous structure (hereinafter referred to as an amorphous oxide semiconductor). Further, the oxide semiconductor film may include a CAAC-OS. Furthermore, the oxide semiconductor film may include an amorphous oxide semiconductor and an oxide semiconductor having a crystal grain. Described below are a CAAC-OS and a microcrystalline oxide semiconductor as typical examples.

<CAAC-OS>

The CAAC-OS film is one of oxide semiconductor films having a plurality of crystal parts. The crystal parts included in the CAAC-OS film each have c-axis alignment. In a plan TEM image, the area of the crystal parts included in the CAAC-OS film is greater than or equal to 2500 nm², preferably greater than or equal to 5 μm², further preferably greater than or equal to 1000 μm². Furthermore, in a cross-sectional TEM image, when the proportion of the crystal parts is greater than or equal to 50%, preferably greater than or equal to 80%, further preferably greater than or equal to 95% of the CAAC-OS film, the CAAC-OS film is a thin film having physical properties similar to those of a single crystal.

In a transmission electron microscope (TEM) image of the CAAC-OS film, it is difficult to clearly observe a boundary between crystal parts, that is, a grain boundary. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflecting unevenness of a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged parallel to the formation surface or the top surface of the CAAC-OS film. In this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

Note that in an electron diffraction pattern of the CAAC-OS film, spots (luminescent spots) having alignment are shown.

From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. When the CAAC-OS film is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (00x) plane (x is an integral number) of the In—Ga—Zn oxide crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the In—Ga—Zn oxide crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of In—Ga—Zn oxide, six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when 0 scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.

Note that the crystal is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of the formation surface or a normal vector of the top surface of the CAAC-OS film. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depending on regions.

Note that when the CAAC-OS film is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal part having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ not appear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor film, such as silicon, disturbs the atomic arrangement of the oxide semiconductor film by depriving the oxide semiconductor film of oxygen and causes a decrease in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a decrease in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor film might serve as a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density of defect states. In some cases, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defect states is low (the amount of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor including the oxide semiconductor film rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has little variation in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released, and might behave like fixed electric charge. Thus, the transistor which includes the oxide semiconductor film having high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small.

<Microcrystalline Oxide Semiconductor>

In an image obtained with the TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor film in some cases. In most cases, a crystal part in the microcrystalline oxide semiconductor film is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. An oxide semiconductor film including nanocrystal (nc), which is a microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm, is specifically referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In an image obtained with TEM, a crystal grain boundary cannot be found clearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor film depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Further, a halo pattern is shown in a selected-area electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter larger than the diameter of a crystal part (e.g., larger than or equal to 50 nm). Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) close to or smaller than the diameter of a crystal part. Further, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are observed in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots is shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularity as compared to an amorphous oxide semiconductor film. Therefore, the nc-OS film has a lower density of defect states than an amorphous oxide semiconductor film. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film; hence, the nc-OS film has a higher density of defect states than the CAAC-OS film.

<Oxide Semiconductor Film and Oxide Conductor Film>

Next, the temperature dependence of conductivity of a film formed with an oxide semiconductor (hereinafter referred to as an oxide semiconductor film (OS)) and that of a film formed with an oxide conductor (hereinafter referred to as an oxide conductor film (OC)), which can be used for the pixel electrode 19 b, will be described with reference to FIG. 26. In FIG. 26, the horizontal axes represent measurement temperature (the lower horizontal axis represents 1/T and the upper horizontal axis represents T), and the vertical axis represents conductivity (1/ρ). Measurement results of the oxide semiconductor film (OS) are plotted as triangles, and measurement results of the oxide conductor film (OC) are plotted as circles.

Note that a sample including the oxide semiconductor film (OS) was prepared by forming a 35-nm-thick In—Ga—Zn oxide film over a glass substrate by a sputtering method using a sputtering target with an atomic ratio of In:Ga:Zn=1:1:1.2, forming a 20-nm-thick In—Ga—Zn oxide film over the 35-nm-thick In—Ga—Zn oxide film by a sputtering method using a sputtering target with an atomic ratio of In:Ga:Zn=1:4:5, performing heat treatment in a 450° C. nitrogen atmosphere and then performing heat treatment in a 450° C. atmosphere of a mixed gas of nitrogen and oxygen, and forming a silicon oxynitride film over the oxide films by a plasma CVD method.

A sample including the oxide conductor film (OC) was prepared by forming a 100-nm-thick In—Ga—Zn oxide film over a glass substrate by a sputtering method using a sputtering target with an atomic ratio of In:Ga:Zn=1:1:1, performing heat treatment in a 450° C. nitrogen atmosphere and then performing heat treatment in a 450° C. atmosphere of a mixed gas of nitrogen and oxygen, and forming a silicon nitride film over the oxide film by a plasma CVD method.

As can be seen from FIG. 26, the temperature dependence of conductivity of the oxide conductor film (OC) is lower than the temperature dependence of conductivity of the oxide semiconductor film (OS). Typically, the range of variation of conductivity of the oxide conductor film (OC) at temperatures from 80 K to 290 K is from more than −20% to less than +20%. Alternatively, the range of variation of conductivity at temperatures from 150 K to 250 K is from more than −10% to less than +10%. In other words, the oxide conductor is a degenerate semiconductor and it is suggested that the conduction band minimum agrees with or substantially agrees with the Fermi level. Therefore, the oxide conductor film (OC) can be used for a resistor, a wiring, an electrode, a pixel electrode, a common electrode, or the like.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 9

In the method for manufacturing any of the transistors described in the above embodiments, after the conductive films 21 a and 21 b functioning as a source electrode and a drain electrode are formed, the oxide semiconductor film 19 a may be exposed to plasma generated in an oxidizing atmosphere, so that oxygen may be supplied to the oxide semiconductor film 19 a. Atmospheres of oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples of oxidizing atmospheres. Further, in the plasma treatment, the oxide semiconductor film 19 a is preferably exposed to plasma generated with no bias applied to the substrate 11 side. Consequently, the oxide semiconductor film 19 a can be supplied with oxygen without being damaged; accordingly, the amount of oxygen vacancies in the oxide semiconductor film 19 a can be reduced. Moreover, impurities, e.g., halogen such as fluorine or chlorine, remaining on a surface of the oxide semiconductor film 19 a due to the etching treatment can be removed. The plasma treatment is preferably performed while heating is performed at a temperature higher than or equal to 300° C. Oxygen in the plasma is bonded to hydrogen contained in the oxide semiconductor film 19 a to form water. Since the substrate is heated, the water is released from the oxide semiconductor film 19 a. Consequently, the amount of hydrogen and water in the oxide semiconductor film 19 a can be reduced.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments.

Embodiment 10

In this embodiment, structural examples of electronic appliances each using a display device of one embodiment of the present invention will be described. In addition, in this embodiment, a display module using a display device of one embodiment of the present invention will be described with reference to FIG. 15.

In a display module 8000 in FIG. 15, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight unit 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002. Note that the backlight unit 8007, the battery 8011, the touch panel 8004, and the like are not provided in some cases.

The display device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and may be formed so as to overlap with the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 to form an optical touch panel. An electrode for a touch sensor may be provided in each pixel of the display panel 8006 so that a capacitive touch panel is obtained.

The backlight unit 8007 includes a light source 8008. The light source 8008 may be provided at an end portion of the backlight unit 8007 and a light diffusing plate may be used.

The frame 8009 protects the display panel 8006 and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 16A to 16D are each an external view of an electronic appliance including a display device of one embodiment of the present invention.

Examples of electronic appliances are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone (also referred to as a cellular phone or a cellular phone device), a portable game machine, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like.

FIG. 16A illustrates a portable information terminal including a main body 1001, a housing 1002, display portions 1003 a and 1003 b, and the like. The display portion 1003 b is a touch panel. By touching a keyboard button 1004 displayed on the display portion 1003 b, a screen can be operated, and text can be input. It is needless to say that the display portion 1003 a may be a touch panel. A liquid crystal panel or an organic light-emitting panel is manufactured by using any of the transistors described in the above embodiments as a switching element and used in the display portion 1003 a or 1003 b, whereby a highly reliable portable information terminal can be provided.

The portable information terminal illustrated in FIG. 16A can have a function of displaying a variety of kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a function of operating or editing data displayed on the display portion, a function of controlling processing by a variety of kinds of software (programs), and the like. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing.

The portable information terminal illustrated in FIG. 16A may transmit and receive data wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an e-book server.

FIG. 16B illustrates a portable music player, which includes in a main body 1021, a display portion 1023, a fixing portion 1022 with which the portable music player can be worn on the ear, a speaker, an operation button 1024, an external memory slot 1025, and the like. A liquid crystal panel or an organic light-emitting panel is manufactured using any of the transistors described in the above embodiments as a switching element, and used in the display portion 1023, whereby a highly reliable portable music player can be provided.

Furthermore, when the portable music player illustrated in FIG. 16B has an antenna, a microphone, or a wireless communication function and is used with a mobile phone, a user can talk wirelessly and hands-freely on the phone while driving a car or the like.

FIG. 16C illustrates a mobile phone, which includes two housings, a housing 1030 and a housing 1031. The housing 1031 includes a display panel 1032, a speaker 1033, a microphone 1034, a pointing device 1036, a camera lens 1037, an external connection terminal 1038, and the like. The housing 1030 is provided with a solar cell 1040 for charging the mobile phone, an external memory slot 1041, and the like. In addition, an antenna is incorporated in the housing 1031. Any of the transistors described in the above embodiments is used in the display panel 1032, whereby a highly reliable mobile phone can be provided.

Further, the display panel 1032 includes a touch panel. A plurality of operation keys 1035 displayed as images is indicated by dashed lines in FIG. 16C. Note that a boosting circuit by which voltage output from the solar cell 1040 is increased to be sufficiently high for each circuit is also included.

In the display panel 1032, the direction of display is changed as appropriate depending on the application mode. Further, the mobile phone is provided with the camera lens 1037 on the same surface as the display panel 1032, and thus it can be used as a video phone. The speaker 1033 and the microphone 1034 can be used for videophone calls, recording and playing sound, and the like as well as voice calls. Moreover, the housings 1030 and 1031 in a state where they are developed as illustrated in FIG. 16C can shift, by sliding, to a state where one is lapped over the other. Therefore, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried around.

The external connection terminal 1038 can be connected to an AC adaptor and a variety of cables such as a USB cable, whereby charging and data communication with a personal computer or the like are possible. Further, by inserting a recording medium into the external memory slot 1041, a larger amount of data can be stored and moved.

Further, in addition to the above functions, an infrared communication function, a television reception function, or the like may be provided.

FIG. 16D illustrates an example of a television set. In a television set 1050, a display portion 1053 is incorporated in a housing 1051. Images can be displayed on the display portion 1053. Moreover, a CPU is incorporated in a stand 1055 for supporting the housing 1051. Any of the transistors described in the above embodiments is used in the display portion 1053 and the CPU, whereby the television set 1050 can have high reliability.

The television set 1050 can be operated with an operation switch of the housing 1051 or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

Note that the television set 1050 is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the television set is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed.

Further, the television set 1050 is provided with an external connection terminal 1054, a storage medium recording and reproducing portion 1052, and an external memory slot. The external connection terminal 1054 can be connected to various types of cables such as a USB cable, and data communication with a personal computer or the like is possible. A disk storage medium is inserted into the storage medium recording and reproducing portion 1052, and reading data stored in the storage medium and writing data to the storage medium can be performed. In addition, an image, a video, or the like stored as data in an external memory 1056 inserted into the external memory slot can be displayed on the display portion 1053.

Further, in the case where the off-state leakage current of the transistor described in the above embodiments is extremely small, when the transistor is used in the external memory 1056 or the CPU, the television set 1050 can have high reliability and sufficiently reduced power consumption.

This embodiment can be combined with any of the other embodiments disclosed in this specification as appropriate.

Example 1

In this example, distribution of transmittance of a pixel included in a liquid crystal display device according to one embodiment of the present invention was evaluated by calculation.

First, samples used in this example are described.

FIG. 17A is a top view of Sample 1 which is a comparative example. The area of a pixel in Sample 1 includes a scan line 201 and a common line 203 which extend in a horizontal direction, a signal line 205 which extends in a vertical direction (a direction perpendicular to the scan line and the common line), and a region surrounded by these lines. One pixel is 84 μm long and 28 μm wide.

Sample 1 includes a common electrode 207 which is placed inside a region surrounded by the above-mentioned lines and a signal line of a horizontally adjacent pixel and electrically connected to the common line 203, and a comb-like pixel electrode 209 placed over the common electrode 207. Teeth of the pixel electrode 209 extend in a direction intersecting with the signal line 205. In Sample 1, the pixel is provided with a transistor including a gate electrode electrically connected to the scan line 201, a semiconductor film 211 which overlaps with the gate electrode with a gate insulating film provided therebetween and is formed through the same process as the common electrode 207, a source electrode electrically connected to the semiconductor film 211 and the signal line 205, and a drain electrode 213 electrically connected to the semiconductor film 211 and the pixel electrode 209.

FIG. 17B is a top view of Sample 2 which is one embodiment of the present invention. The area of a pixel in Sample 2 includes a scan line 221 extending in a horizontal direction, a signal line 225 extending in a vertical direction, and a region surrounded by these lines. One pixel is 84 μm long and 28 μm wide.

Sample 2 includes a pixel electrode 229 which is placed inside a region surrounded by the above-mentioned lines, a signal line of a horizontally adjacent pixel, and a scan line of a vertically adjacent pixel, and a common electrode 227 placed over the pixel electrode 229. The common electrode 227 includes stripe regions extending in a direction intersecting with the signal line 225. In Sample 2, the pixel is provided with a transistor including a gate electrode electrically connected to the scan line 221, a semiconductor film 231 which overlaps with the gate electrode with a gate insulating film provided therebetween and is formed through the same process as the pixel electrode 229, a source electrode electrically connected to the semiconductor film 231 and the signal line 225, and a drain electrode 233 electrically connected to the semiconductor film 231 and the pixel electrode 229. The transistor 102 described in Embodiment 2 and illustrated in FIG. 5 can be referred to for the cross-sectional shape of the transistor.

Sample 1 and Sample 2 were prepared in the above manner. Transmittance of liquid crystals in the pixels of Sample 1 and Sample 2 can be controlled by a horizontal electric field applied between the pixel electrode and the common electrode.

Next, the transmittance of Sample 1 and Sample 2 were calculated. The calculation was performed using LCDMaster 3D (produced by SHINTECH, Inc.) in an FEM-Static mode. In the calculation, the size was 84 μm long, 28 μm wide, and 4 μm deep (high), and the boundary condition was a periodic boundary condition. The gate electrode was 200 nm thick, the gate insulating film was 400 nm thick, the signal line was 300 nm thick, and an interlayer insulating film was 500 nm thick in each of Sample 1 and Sample 2. In Sample 1, the common electrode was 0 nm thick, a nitride insulating film between the common electrode and the pixel electrode was 100 nm thick, and the pixel electrode was 100 nm thick. In Sample 2, the pixel electrode was 0 nm thick, a nitride insulating film between the pixel electrode and the common electrode was 100 nm thick, and the common electrode was 100 nm thick. The rubbing direction of the liquid crystal was 85°, the twist angle was 0°, and the pretilt angle was 3°. Note that the common electrode of Sample 1 and the pixel electrode of Sample 2 were each 0 nm thick in order to reduce calculation load. The distribution of transmittance in the case where −9 V was applied to the scan line, 0 V was applied to the common line, and 6 V was applied to the signal line and the pixel electrode under the above conditions was evaluated.

The distribution of transmittance is expressed by grayscale; a whiter region has higher transmittance. FIG. 17C shows the distribution of transmittance of Sample 1 and FIG. 17D shows the distribution of transmittance of Sample 2.

It is found that regions with high transmittance were formed in Sample 1 and Sample 2. In particular, a region with high transmittance was formed in a wide area in the pixel in Sample 2. This is because the common electrode of Sample 2 does not include a region extending in a direction parallel to the signal line and an electric field between the pixel electrode and the common electrode is generated in a wider region in Sample 2 than in Sample 1.

Thus, Sample 2 is an effective structure for a liquid crystal display device with low power consumption.

Example 2

In this example, light leakage in a black display region when white and black are displayed in adjacent pixels in a liquid crystal display device according to one embodiment of the present invention was evaluated by calculation.

First, samples used in this example are described.

FIG. 18A is a top view of Sample 3. The area of a pixel of Sample 3 includes a scan line 241 extending in a horizontal direction, a signal line 243 extending in a vertical direction, and a region surrounded by these lines. The area of two horizontally adjacent pixels is 49.5 μm long and 30 μm wide.

Sample 3 includes a pixel electrode 249 placed inside a region surrounded by the above lines, a signal line of a horizontally adjacent pixel, and a scan line of a vertically adjacent pixel, and a common electrode 247 placed over the pixel electrode 249. The common electrode 247 includes stripe regions extending in a direction intersecting with the signal line 243. In Sample 3, the pixel is provided with a transistor including a gate electrode electrically connected to the scan line 241, a semiconductor film 251 which overlaps with the gate electrode with a gate insulating film provided therebetween and is formed through the same process as the pixel electrode 249, a source electrode electrically connected to the semiconductor film 251 and the signal line 243, and a drain electrode 253 electrically connected to the semiconductor film 251 and the pixel electrode. The transistor 102 described in Embodiment 2 and illustrated in FIG. 5 can be referred to for the cross-sectional shape of the transistor.

FIG. 18B is a top view of Sample 4. Sample 4 has a structure similar to the structure of Sample 3 except for the shapes of the drain electrode and the common electrode. Specifically, in Sample 4, a drain electrode 263 has an L shape and overlaps with an end portion of the pixel electrode 249. Thus, the influence of an electric field generated between the scan line 241 and the pixel electrode 249 is reduced. Furthermore, a common electrode 267 is connected to a vertically adjacent pixel across the scan line 241, whereby the influence of an electric field generated between the scan line 241 and the pixel electrode 249 is reduced.

Sample 3 and Sample 4 were prepared in the above manner. Transmittance of liquid crystal elements in the pixels of Sample 3 and Sample 4 can be controlled by a horizontal electric field applied between the pixel electrode and the common electrode.

Next, the transmittance of Sample 3 and Sample 4 were calculated. The calculation was performed using LCDMaster 3D (produced by SHINTECH, Inc.) in an FEM-Static mode. In the calculation, the size was 49.5 μm long, 30 μm wide, and 4 μm deep (high), and the boundary condition was a periodic boundary condition. The gate electrode was 200 nm thick, the gate insulating film was 400 nm thick, the pixel electrode was 0 nm thick, the signal line was 300 nm thick, an interlayer insulating film was 500 nm thick, and the common electrode was 100 nm thick in each of Sample 3 and Sample 4. A nitride insulating film between the pixel electrode and the common electrode was 100 nm thick. The rubbing direction of the liquid crystal was 90°, the twist angle was 0°, and the pretilt angle was 3°. Note that the pixel electrode was 0 nm thick in order to reduce calculation load. The distribution of transmittance in the case where −9 V was applied to the scan line, 0 V was applied to the common line, 6 V was applied to the signal line and the pixel electrode of the left pixel, and 0 V was applied to the signal line and the pixel electrode of the right pixel under the above conditions was evaluated.

The distribution of transmittance is expressed by grayscale; a whiter region has higher transmittance. FIG. 18C shows the distribution of transmittance of Sample 3 and FIG. 18D shows the distribution of transmittance of Sample 4.

In each of Sample 3 and Sample 4, white display and black display were observed on the left pixel and on the right pixel, respectively. In the black display of Sample 3, a region with high transmittance (light leakage) was partly observed. In contrast, in the black display of Sample 4, a region with high transmittance was not observed in the entire pixel. Since the drain electrode 263 has an L shape and overlaps with the end portion of the pixel electrode 249 in Sample 4, an electric field between the scan line and the pixel electrode is less likely to be generated in Sample 4 than in Sample 3, and light leakage in black display is reduced.

Thus, Sample 4 is an effective structure for a liquid crystal display device with high contrast.

This application is based on Japanese Patent Application serial no. 2013-177345 filed with Japan Patent Office on Aug. 28, 2013, and Japanese Patent Application serial no. 2014-047301 filed with Japan Patent Office on Mar. 11, 2014, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A display device comprising: a first pixel electrode; a second pixel electrode; a first line; a second line; a third line; a first transistor electrically connected between the first pixel electrode and the second line; a second transistor electrically connected between the second pixel electrode and the third line; a first insulating film over the first transistor, the second transistor, the first pixel electrode and the second pixel electrode; and a common electrode which is over the first insulating film and overlaps with the first line, wherein the first line and the second line intersect with each other, wherein the first transistor and the second transistor each include part of the first line as a gate electrode, wherein a channel region of the first transistor is formed in a first semiconductor film and a channel region of the second transistor is formed in a second semiconductor film, wherein the first semiconductor film and the second semiconductor film are over the first line with a gate insulating film interposed therebetween and in contact with the gate insulating film, wherein the first pixel electrode and the second pixel electrode are over and in contact with the gate insulating film, wherein the common electrode does not overlap with the channel region of the first transistor and the channel region of the second transistor, wherein the common electrode comprises stripe regions, and wherein the stripe regions extend in a direction intersecting with the second line and the third line and overlap with each of the first pixel electrode and the second pixel electrode.
 2. The display device according to claim 1, wherein one of a source electrode and a drain electrode of the first transistor comprises a region which extends parallel to the first line and is in contact with the first pixel electrode, and wherein one of a source electrode and a drain electrode of the second transistor comprises a region which extends parallel to the first line and is in contact with the second pixel electrode.
 3. The display device according to claim 1, wherein the stripe regions and at least one of the second line and the third line intersect with each other at an angle larger than or equal to 70° and smaller than or equal to 110°.
 4. The display device according to claim 1, wherein the first semiconductor film, the second semiconductor film, the first pixel electrode and the second pixel electrode each comprise an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide, and wherein M represents Al, Ga, Y, Zr, La, Ce, or Nd.
 5. The display device according to claim 4, further comprising a second insulating film between the first insulating film and each of the first transistor and the second transistor, wherein the first insulating film is a nitride insulating film in contact with the first pixel electrode and the second pixel electrode, and wherein the second insulating film is an oxide insulating film in contact with the first semiconductor film and the second semiconductor film.
 6. The display device according to claim 5, wherein concentration of hydrogen in each of the first pixel electrode and the second pixel electrode is greater than or equal to 10 times concentration of hydrogen in each of the first semiconductor film and the second semiconductor film.
 7. A display device comprising: a first line; a second line; a third line; a first pixel electrode; a second pixel electrode; a first transistor electrically connected between the first pixel electrode and the second line; a second transistor electrically connected between the second pixel electrode and the third line; a first insulating film over the first pixel electrode, the second pixel electrode, the first transistor, and the second transistor; and a common electrode which is over the first insulating film and overlapping with the first line, wherein the first transistor and the second transistor each include part of the first line as a gate electrode, wherein the first transistor comprises a first semiconductor film and the second transistor comprises a second semiconductor film, wherein the first semiconductor film and the second semiconductor film are over the first line with a gate insulating film interposed therebetween and in contact with the gate insulating film, wherein the first pixel electrode and the second pixel electrode are over and in contact with the gate insulating film, wherein the first line and each of the second line and the third line intersect with each other, wherein the common electrode comprises stripe regions, and wherein the stripe regions extend in a direction intersecting with the second line and the third line and overlap with the first pixel electrode and the second pixel electrode.
 8. The display device according to claim 7, wherein one of a source electrode and a drain electrode of the first transistor comprises a region which extends parallel to the first line and is in contact with the first pixel electrode, and wherein one of a source electrode and a drain electrode of the second transistor comprises a region which extends parallel to the first line and is in contact with the second pixel electrode.
 9. The display device according to claim 7, wherein the stripe regions and at least one of the second line and the third line intersect with each other at an angle larger than or equal to 70° and smaller than or equal to 110°.
 10. The display device according to claim 7, wherein the first semiconductor film, the second semiconductor film, the first pixel electrode and the second pixel electrode each comprise an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide, and wherein M represents Al, Ga, Y, Zr, La, Ce, or Nd.
 11. The display device according to claim 10, further comprising a second insulating film between the first insulating film and each of the first transistor and the second transistor, wherein the first insulating film is a nitride insulating film in contact with the first pixel electrode and the second pixel electrode, and wherein the second insulating film is an oxide insulating film in contact with the first semiconductor film and the second semiconductor film.
 12. The display device according to claim 11, wherein concentration of hydrogen in each of the first pixel electrode and the second pixel electrode is greater than or equal to 10 times concentration of hydrogen in each of the first semiconductor film and the second semiconductor film. 